The effect of temperature and of the initial molar isopropanol-isobutene ratio, R-A/O, on byproducts formation in isopropyl tert-butyl ether (IPTBE) synthesis in the presence of the resin Bayer K-2631 were determined. The formation of diisopropyl ether, diisobutene (2,4,4-trimethyl-1-pentene, and 2,4,4-trimethyl-2-pentene isomers), and tert-butanol was studied in the temperature range 30-80 degrees C, with R-A/O ranging from 1.0 to 4.3. The formation of tert-butanol is fast, and it reaches quickly the chemical equilibrium. The extent of the reaction is therefore limited by the amount of available water. The reactions forming 2,4,4-trimethyl-1-pentene, and 2,4,4-trimethyl-2-pentene take place gradually in the whole temperature range explored, whereas diisopropyl ether formation only occurs over 60 degrees C once the reaction of IPTBE synthesis is close to equilibrium. These reactions are favoured by high temperatures. Low R-A/O values favour isobutene dimerization and tert-butanol formation. The effect of R-A/O on the reaction forming diisopropyl ether is negligible.