Cities Service Co. yesterday announced that its second-quarter earnings rose 39 1/2 percent, Union Oil Co. of California posted a 52 percent increase in the same period, and Ashland Oil Co. reported a sharp drop in its third-quarter profits.

On Tuesday, Exxon Corp., America's largest oil company, said its second-quarter earnings increased 24 percent over a year ago, and Standard Oil Co. (Ohio), ranked No. 14 among U.S. refiners, posted a 124 percent jump.

Cities Service, the nation's 17th-largest oil company, earned $107 million ($1.29 a share), up form $76.7 million (92 cents) in the 1979 second quarter. Revenues rose 28 percent to $1.84 million versus $1.44 million.

Cities Service said it recorded improved profitability in most of its businesses. Domestic results were hurt, however, by the "windfall profits" tax and higher exploration costs. International earnings benefited from the syn-crude tar sand operation in Canada.

First-half profits were $277.1 million ($3.33), up 79 percent from $155.2 million ($1.87) a year earlier. Revenues rose 39 percent to $4.04 billion from $2.9 billion.

Ashland, the 15th-largest refiner, earned $46.5 million ($1.54 a share) in its third-quarter ended in June 30, down from $395.4 million ($12.35) in the same period last year. Revenues were up 18 percent to $2.06 billion from $1.74 billion.

Ashland sold a number of subsidiaries last year which swelled its earnings in the 1979 third quarter by $345.1 million ($10.83).

Asland's nine-month earnings were $160.9 million ($5.65), compared with $473.4 million ($13.66). Revenues rose to $5.97 billion from $4.57 billion.

Union Oil of California reported its second-quarter earnings rose to $194.8 million ($2.24 a share) from $128.2 million ($1.48) in the 1979 second quarter. Revenues were up 47 percent to $2.75 billion from $1.87 billion.

Union, the 13th-largest refiner, said its second-quarter profit increase reflected higher oil and gas production earnings in the United States and abroad, bigger investment tax credits and improvement in other major operations.

First-half net income was $347.7 million ($4.01), up 43 percent from $242.3 million ($2.78) a year earlier. Revenues improved 41 percent to $5.13 billion from $3.63 billion.

UAL Inc., the nation's second-largest transportation company and the holding firm for United Airlines, had an operating loss of $23.9 million in the second quarter, but tax adjustments and capital gains resulted in net income of $24.76 million (83 cents a share).

That contrasted with a loss of $47.36 million a year earlier, when the company was hit by a prolonged strike.

Revenues rose to $1.23 billion from $465.24 million a year earlier.

The first half produced a loss of $15.56 million on revenues of $2.39 billion, compared with a loss of $44.37 million a year ago on revenues of $1.44 billion.

Eastman Kodak Co. yesterday reported earnings increases of 22 percent in the second quarter and 24 percent in the first half.

Kodak earned $272.5 million ($1.69 a share) in the second quarter, up from $222.7 million ($1.38) in the same period a year ago. Sales for the three months were $2.12 billion, up from $1.8 billion last year.

First-half profits were $448.4 million ($3.03), up from $420.23 million ($2.60) a year earlier. Sales for the period increased 24 percent to $4.27 billion from $3.45 billion in 1979.

American Express Co., the giant travel service and credit card concern, said yesterday that its net income in the second quarter rose 7 percent to $98.2 million ($1.37 a share) from $91.6 million ($1.29) in the comparable 1979 period. Total revenues jumped to $1.36 billion from $1.14 billion.

The company's first-half earnings rose 6.6 percent to $176 million ($2.46) from $165 million ($2.31).

General Foods Corp. yesterday reported earnings of $72.4 million ($1.45 a share) in its first quarter ended in June 28, up 5.1 percent from the $68.9 million ($1.38) reported in the same period a year ago.

General Foods, which is the nation's largest manufacturer of packaged foods, said net sales of $1.8 billion were 18.3 percent higher than the $1.5 billion in the first quarter of its last fiscal year.