H-1 broad-line (4 K) and MAS (room temperature) NMR have been used to study the acid strength of two amorphous silica-aluminas interacting or not with adsorbed water. The study is more difficult than for zeolites, because the acidic SiO(H)Al bridges are reversibly destroyed by dehydration. However, an acidity coefficient value (H3O+ concentration per Bronsted acid site when one water molecule interacts with each Bronsted site) of 0.34 +/- 10% has been determined. This value is equal to that obtained for H-faujasite and H-mordenite samples with Si/Al ratios high enough for maximum acid strength.