The effect of temperature and of the initial molar methanol-isobutene ratio on by-products formation in methyl tert-butyl ether synthesis catalyzed by a macroporous acidic resin were determined. The presence of dimethyl ether, methyl sec-butyl ether and diisobutene was studied in the temperature range 50-82 degrees C, using an initial molar methanol-isobutene ratio ranging from 1.0 to 3.5. The by products formation was favoured by high temperatures. A low initial molar methanol-isobutene ratio favoured methyl sec-butyl ether and diisobutene formation, whereas high molar ratios favoured dimethyl ether formation. Results were explained by reaction mechanisms and compared with literature data.