State-run oil monopoly Petroleos Mexicanos has called a tender for two desulfurization plants as part of a program to improve the quality of Mexican gasoline, the company said Thursday.

The tender calls for one plant at the Cadereyta refinery and the other at the Madero refinery to produce ultra-low sulfur gasoline. Interested companies have until June 11 to present bids. Work is expected to start on Aug. 10 and last 3 1/2 years.

When Pemex announced its clean fuels program two years ago it said it would invest close to $3 billion on 22 new plants at its six refineries. Pemex did not say how much it plans to spend on the two sulfur removal plants.

Last fall, Pemex rolled out a similar tender for two desulfurization plants, one at the Tula refinery and the other at the Salamanca refinery. On Thursday, Pemex said it will announce a winner for the Salamanca and Tula work in April.

The companies competing for the Tula and Salamanca work include Samsung Engineering and ICA Fluor, a joint venture of Mexico's Empresas ICA (ICA) and Fluor Corp. (FLR).

Pemex plans to lift domestic crude processing at its existing six refineries to 1.8 million barrels a day by 2016, up from around 1.3 million barrels a day at present. Pemex also plans to build a new refinery, but has not yet picked a location.

-By Peter Millard, Dow Jones Newswires; 5255-5001-5724; peter.millard@dowjones.com