Side-chain carboxyl and carbonyl groups play a major role in protein interactions and enzyme catalysis. A series of C-13 relaxation experiments is introduced to study the dynamics of carboxyl and carbonyl groups in protein side chains on both fast (sub-ns) and slower (mu s-ms) time scales. This approach is illustrated on the protein calbindin D-9k. Fast dynamics features correlate with hydrogen- and ion-binding patterns. We also identify chemical dynamics on mu s time scales in solvent-exposed carboxyl groups, most probably due to exchange between the carboxylate and carboxylic acid forms.