Washington Post reporter Patrick Tyler yesterday defended two 1979 articles he wrote about the business dealings of Mobil Oil Corp. president William P. Tavoulareas and his son Peter and cited numerous sources, records and Mobil statements as the basis for his stories.

Tyler, testifying for the second time in the Tavoulareases' $50-million libel suit against The Post, said he "absolutely" stands by the stories and conceded only that he had one date wrong in the Nov. 30, 1979, article and had understated the portion of the London-based shipping management firm owned by the younger Tavoulareas.

Under questioning by Irving Younger, one of the newspaper's attorneys, Tyler described to the six-member federal court jury hearing the case the source material he used when he wrote each of the 29 paragraphs in the two stories that the Tavoulareases allege were libelous.

Tyler said he relied on confidential sources as a basis for some assertions in his stories, although the chief sources have come out during months of preparation for the current trial. In other instances, he said, he quoted verbatim from statements made to him in on-the-record interviews or by various Mobil officials.

The reporter recounted six sources for the first paragraph of the Nov. 30 story, an assertion that the elder Tavoulareas "set up his son" in the shipping firm, Atlas Maritime Co., and that the firm "has since done millions of dollars in business operating Mobil-owned ships under exclusive, no-bid contracts."

In testimony earlier in the 14-day-old trial, the Tavoulareases denied that the Mobil president urged George Comnas, the first managing director of Atlas, to include Peter as a partner.

Tyler said that Comnas gave him "the most inside account" of the Tavoulareas transactions. Various Mobil officials have testified at the trial that Comnas proved to be an incompetent Atlas chief and eventually agreed to resign in 1975. Tyler testified that Comnas' "credibility to me was unchallenged," in part because Mobil officials originally recommended that he be hired at Atlas.

Numerous references have been made to Comnas, a former Exxon official, at the trial, but the newspaper's lawyers expect to rest their case today without calling Comnas to appear as a witness.

Before recalling Tyler to the witness stand, lawyers for The Post read into the trial record testimony given elsewhere by two former Mobil directors, former General Electric Corp. chairman Fred J. Borch and former IBM president Albert L. Williams. Both said they had raised questions with Mobil officials about the arrangements under which the oil company is a partner in a shipping firm, the Saudi Maritime Co. (Samarco), whose shipping operations are in turn managed by Atlas.

Borch said he was concerned about the "appearance of a conflict of interest," but eventually was "perfectly satisfied" with Mobil Board Chairman Rawleigh Warner Jr.'s "explanation that there was no conflict."

Another witness, former Mobil executive Frederick L. Morefield, testified that he raised questions about the Mobil-Samarco-Atlas arrangement before quitting the company in 1975 and later wrote a letter about it to some of Mobil's directors. He characterized the elder Tavoulareas as an "extraordinarily bright" business executive, but also as one who is "vindictive, very dominant."

Tavoulareases' attorneys had unsuccessfully opposed allowing Morefield to testify. But they did succeed yesterday in keeping a critical Securities and Exchange Commission report of the Tavoulareas dealings out of the court record. The commission's staff concluded last year that Mobil's disclosure of the transactions was incomplete, but the commission took no enforcement action.