Equilibria constants for the simultaneous liquid-phase syntheses of methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE) were determined experimentally in the temperature range 313-353 K at 1.6 MPa. To achieve etherification equilibria, the macroporous sulfonic add resin Bayer K=2631 was used as the catalyst. MTBE reached the equilibrium faster than ETBE. The progress of the reactions toward the respective equilibrium was determined using different particle sizes ranging from 0.1 to 1 mm at 50 degrees C. Catalyst effectiveness factors greater than unity were observed for the particle sizes of 0.4-0.63 mm.