Recent developments on acidity characterization of solid acid catalysts, specifically those invoking 31P solid-state nuclear magnetic resonance (SSNMR) spectroscopy using phosphorus-containing molecules as probes has been summarized. In particular, various 31P SSNMR approaches using trimethylphosphine (TMP), phosphines, diphosphines, and trialkylphosphine oxides (R3PO) will be introduced and their practical applications for characterization of important qualitative and quantitative features, viz. type, distribution, accessibility (location/proximity), concentration (amount), and strength of acid sites in various solid acids are illustrated. This is an updated version of the original review (Annu. Rep. NMR Spectr. 81 (2014) 47–108).