Temperature-programmed reduction (TPR) has been applied to study the reduction behavior of chromium oxide supported on pure zirconia and on lanthana-containing zirconia. The results revealed influences of support nature and chromium loading on the state of the supported chromium oxide. With catalysts containing lanthana and only low amounts of chromium, the decrease of the chromium oxidation state during TPR corresponded to the reduction of Cr6+ to Cr3+. With lanthana-free catalysts, lower changes of oxidation state have been found, hinting on thermal instability of higher valent chromium species during thermal catalyst pretreatment. Chromium oxide supported on pure zirconia is reduced at temperatures between 340 and 370 degrees C and is not quantitatively reoxidized by an oxygen treatment. Chromium oxide on lanthana-zirconia is reduced at higher temperatures between 370 and 415 degrees C and is completely reoxidable at Cr loadings less than or equal to 4.0 wt.%. The results suggest that lanthana strengthens the interaction between the zirconia surface and chromium species and, therefore, stabilizes a dispersed state of chromium species.