He used to be president of one of the world's biggest companies, a friend of Vice President Bush and a contender for the presidency of Mexico. But now Jorge Diaz Serrano is marking his 43rd month in Mexico City's Reclusurio Sur federal prison awaiting trial on corruption charges.

The spectacular fall of Diaz Serrano does not appear to have broken him. He maintains his innocence with earnestness and pride, even with an occasional touch of wry humor. "If you're a politician, you're bound to have problems of this sort," Diaz Serrano said.

Diaz Serrano's main argument to Dale Van Atta, who recently talked his way into the prison to interview him, was an interesting one. Simply put, it was that he was in a position to steal a lot more than the $34 million in "illicit profit" he is accused of having made as head of PEMEX, the state-owned oil company.

"I handled $45 billion in export and never touched a cent," he explained. "But I could have."

How? "Simple, simple, my dear Watson," Diaz Serrano replied. He could have stolen $300 million a month "without signing any paper," he said.

Acknowledged as the architect of Mexico's short-lived oil boom, Diaz Serrano had already made a fortune in oil contracting when his friend, President Jose Lopez Portillo, asked him to run PEMEX in 1976. Lopez Portillo fired him on June 6, 1981, for dropping the price of Mexican oil without consulting the president.

During his tenure at PEMEX, Diaz Serrano pushed Mexico's oil exports from virtually nothing to 1.5 million barrels a day, doubled refining capacity and tripled oil and petrochemical production.

In fact, just before his fall, Diaz Serrano was emerging as a likely choice to succeed Lopez Portillo, his close friend and weekly tennis partner.

Lopez Portillo's successor, President Miguel de la Madrid, ran on an anticorruption platform, and not long after he took office he had Diaz Serrano arrested. By most accounts, Diaz Serrano was the most convenient sacrificial lamb to lend substance to the new president's campaign promise.

Diaz Serrano knows it, he explained, because a Belgian businessman, who offered to prove there was no fraud in the deal for which Diaz Serrano was jailed, was told by a high Mexican official that it didn't matter. "He told him that this was a political problem of Jorge Diaz Serrano, and he was not to be concerned about it," Diaz Serrano said.

This is not to say that there wasn't widespread thievery during Diaz Serrano's tenure at PEMEX. In 1979 alone, for example, 317 million barrels of oil, worth more than $5 billion, disappeared from the books. Diaz Serrano suggested that the oil had, for the most part, "evaporated" or otherwise had been lost in the refining process.

One point Diaz Serrano is fervent about is denying that he was ever a business partner of Bush, as has been widely reported.

Diaz Serrano is resigned to two more years in the pokey, until the end of the de la Madrid administration. Why? Because Francisco Rojas, the official who told the Belgian businessman that Diaz Serrano is a political prisoner, was just rewarded with the job as head of PEMEX.