Acidity of group 5 metal-substituted H4PW11-M1O40 (M = V, Nb, Ta) Keggin and H7P2W17M1O62 (M = V, Nb, Ta) Wells-Dawson heteropolyacid (HPA) catalysts was measured by NH3-TPD experiment. Acidity of H4PW11M1O40 (M = V, Nb, Ta) and H7P2W17M1O62 (M = V, Nb, Ta) HPA catalysts showed the same trend with respect to polyatom substitution in both series of HPA catalysts. Vapor-phase esterification of acetic acid with ethanol was carried out as a model reaction to correlate the acidity with the acid catalysis of the HPA catalysts. Yield for ethyl acetate (a product formed by acid catalysis of HPA) over H4PW11M1O40 (M = V, Nb, Ta) and H7P2W17M1O62 (M = V, Nb, Ta) HPA catalysts increased with increasing acidity of the HPA catalysts, regardless of the identity of HPA catalyst. Acidity of H4PW11M1O40 (M = V, Nb, Ta) and H7P2W17M1O62 (M = V, Nb, Ta) HPA catalysts could be utilized as a probe of acid catalysis for esterification of acetic acid with ethanol.