Starting from an as-synthesized Na-bearing MCM-41 sample with Si/Al=20, several Cs or K exchanged samples have been obtained. All samples, both washed after exchange to remove the excess alkali–metal salt and unwashed, were characterized by studying in the IR the adsorption of CO at a nominal temperature of 77 K, and of CO2 at room temperature.On unwashed samples, after thermal treatment at 500 °C, a basic alkali-oxide phase and a surface silicate were detected. In contrast, washed samples did not exhibit basic character and only Lewis acidic cationic centers were detected, the strength of which is close to that of alkali-cation exchanged ZSM-5 systems. Unwashed samples were inactive in the isomerization of 1-butene. Washed samples converted 1-butene to cis/trans-2-butene, with a ratio close to one (the presence of isobutene was negligible), as typical of mildly acidic Brønsted centers, which are probably present even after exchange: indeed, a sample of Al-MCM-41 in the protonic form, tested as reference, gave the same reaction with highest activity.