Trimethylsilylpropanoic acid (TMSP or TSP) is a chemical compound containing a trimethylsilyl group. It is used as internal reference in nuclear magnetic resonance for aqueous solvents (e.g. D<sub>2</sub>O). For that use it is often deuterated (3-(trimethylsilyl)-2,2,3,3-tetradeuteropropionic acid or TMSP-d<sub>4</sub>). Other internal references that are frequently used in NMR experiments are DSS and tetramethylsilane.