The acid-base properties of a series of zeolite NaX samples, impregnated with several rare-earth nitrates (Ce, Nd, Eu, Dy, Er, Tm, Yb and Lu) and thermally treated under air, have been tested on the model reaction of 1-butene double bond position isomerization at different temperatures (200-300 degreesC). The activity rises with the reaction temperature increase and the participation of the basic mechanism decreases, while that of the acidic process increases. At 250 degreesC, the initial catalytic activities do not present significant differences among the rare-earth series (0.5-1.5 mmol/(h g)) with the exception of Yb; in this case, the total activity is quite higher (11.5 mmol/(h g)). The basic character of the transformation, evaluated from the cis/trans-2-butene ratio, is superior for part of the impregnated samples than for the rare-earth-free NaX sample, which presents a value of 0.94 for this parameter. The nature of the rare earth does not induce important changes in the basicity among the series, the cis/trans ratio decreasing from 1.9 (Ce) to 1.3 (Lu), with the same exception of the Yb impregnated sample (cis/trans = 2.4); the basic character of this sample is higher than the other one. (C) 2002 Elsevier Science B.V. All rights reserved.