Oxidative dehydrogenation of ethane to ethylene has been studied on V- and Cr-based phosphate catalysts at 550°C, yielding almost exclusively ethylene and CO x . Zirconium hydrogenophosphates in their α or β phase were either submitted to cationic exchange of protons by VO 2 + and Cr 3 + salts or impregnated by such salts and then compared to (VO) 2 P 2 O 7 and CrPO 4 pure phases. VO 2 + and Cr 3 + on the αZr(HPO 4 ) 2 2H 2 O phase were observed to be more selective toward ethylene than the same cations deposited or exchanged on βZr(HPO 4 ) 2 and than the corresponding oxides V 2 O 5 and Cr 2 O 3 either pure (bulk) or deposited on supports as SiO 2 , ZrO 2 or ZrP 2 O 7 . Moreover, (VO) 2 P 2 O 7 (and to a lesser extent CrPO 4 ) were observed to be even more selective toward ethylene at similar conversion levels. However, in terms of cation content per weight, the better yields of ethylene were obtained for αZrCrP and Cr/αZrP samples. The turnover number of ethane conversion per cation accessible was 0.26 min - 1 for both V and Cr cations exchanged or supported on an αZrP phase. Ultraviolet (UV) and electron spin resonance (ESR) spectroscopic techniques, which are very sensitive to the oxidation state and environment symmetry of V and Cr cations, were used in addition to structural and morphological determination techniques such as X-ray diffraction (XRD), infrared (IR), BET, scanning electron microscopy (SEM), etc. Characterization was done after calcination in air at 500/550°C and after catalytic testing at 550°C. All data allowed us to conclude that catalytic features are related to V and Cr local arrangements (small clusters or chain arrangements) and to the counter anion, O 2 - or PO 3 - 4 . The best catalyst for ethane oxidative dehydrogenation consisted of VO 2 + and Cr 3 + chains separated by PO 3 - 4 anions, which are stronger bases in the sense of Pearson than O 2 - anions.