Several zeolite preparations of varying Si/Al ratios were studied with their base-exchange cations in the Cu 2+ and in the Cu + states. Good linear correlations were established between certain lattice vibration frequencies and the lattice aluminum concentration (Al/unit cell) of the Cu-Y preparation. A better fit of the data was obtained with the pore systems filled with H 2 O than with dry samples. These correlations held for the preparations both before reduction and after reoxidation following reduction with CO or H 2 , showing that dealumination had not occurred during this cycle