1. The activation energy of the dehydration of isoptopyl alcohol on two samples of niobium pentoxide with different activities (ɛ=23.4 and 27.0 kcal/M) and on tantalum pentoxide (ɛ=23.8 kcal/M) was determined. The action (reaction velocity) constants and the constants of the logarithmic dependence were determined. 2. It was found that the activity of Nb2O5 depends to a great extent on the temperature of glowing and duration of the glowing to which the oxide has been subjected in connection with its preparation from the metal. The most active sample of Nb2O5 was obtained by glowing metallic Nb at 530° for 1 hr. 3. On the basis of data obtained in X-ray phase analysis, the catalytically more active form of Nb2O5 was found to consist of the low-temperature γ-modification. The high-temperature modification of Nb2O5 proved to be less active. There was no change of the Nb2O5 to another modification during the dehydration of the alcohol under the conditions studied. 4. Ta2O5 prepared under comparable conditions was found to be more active than Nb2O5. 5. It was established that catalysts consisting of pure Nb2O5 or Ta2O5 can be regenerated. The conditions of their regeneration with air were investigated.