Americans' personal income fell 0.1 percent in January, the first decline since last May, the Commerce Department reported yesterday.

The decline last month was attributed to a number of one-time factors. Excluding these, personal income rose 0.2 percent, the Commerce Department said.

The January decline followed a 1.2 percent increase in December and a 0.5 percent rise in November. If the two months since December are averaged, it would just about fit the recent pattern of monthly increases of 0.5 percent, said Robert Ortner, the Commerce Department's chief economist.

"This is not an especially strong report," Ortner said. The increase excluding the extraordinary factors "is on the modest side. It's very difficult to tell on the surface here what the underlying trend is like."

Factors strongly affecting the report were cost-of-living adjustments to several federal-pension and income-support programs, which increased personal income in January, but were offset by higher tax rates and wage base for Social Security contributions, Commerce officials reported.

Another factor was unusually large farm-subsidy payments in December, which plunged in January when government payments became more normal, Ortner said.

Many economists have said that they expect the economy to rebound from the slow rate of growth in the fourth quarter of last year when the increase in gross national product -- the nation's output of goods and services -- was 1.2 percent.

The economists said they expect a better performance largely because of the recent drop in oil prices, which will add to consumers' personal disposable incomes and lead to more consumption and business expansion.

Disposable personal income -- income after taxes -- rose 0.2 percent in January, following a 1.2 percent increase in December and a 0.5 percent rise in November. Personal tax and nontax payments declined largely because of the indexing provision of the Economic Recovery Tax Act of 1981, Commerce reported.

Wages and salaries increased $6.5 billion in January, compared with a $15.2 billion increase in December. Payrolls for industries producing commodities rose $1.8 billion, down from a $6.3 billion increase in December. Service industries' payrolls rose $2.7 billion, down from the $7.1 billion increase the previous month.

Manufacturing payrolls declined $1.1 billion in January, compared with a $4.6 billion increase in December. Average weekly hours and average hourly earnings also fell, but employment increased slightly, Commerce officials reported.

Personal-consumption expenditures -- outlays minus interest paid by consumers to business and personal transfer payments to foreigners -- fell 0.4 percent in January, compared with a 2.1 percent increase in December and a 0.7 percent rise in November.