The effect of aging on the Wichterle-Marek-Trekoval (WMT) solid cationic catalyst for isobutene polymerization has been investigated. It was confirmed that this complex catalyst gives high molecular weight polyisobutenes at high rates at relatively high temperatures. Aging for various times and temperatures the Al(O-s-C4H9)3/BF3component and the Al(O-s-C4H9)3/BF3/TiCl4final catalyst strongly depresses rates of polymerization but only slightly affects the molecular weights produced. Fastest rates and molecular weights are obtained with freshly prepared catalyst whose Ti/Al ratio is ∼ 0.3 at about 30% conversions. Rates and polyisobutene molecular weights obtained by the present authors were higher than those reported by the Czechoslovakian workers. These results are discussed in terms of cationic polymerization mechanism, and an explanation for the reduced rates and molecular weights produced with aged WMT catalysts is proposed.