The conformations of trimethyl phosphite (TMPhite) were studied using matrix isolation infrared spectroscopy. TMPhite was trapped in a nitrogen matrix using an effusive source maintained at two different temperatures (298 and 410 K) and a supersonic jet source. The experimental studies were supported by ab initio computations performed at the B3LYP/6-31++G** level. Computations identified four minima for TMPhite, corresponding to conformers with C-1-(TG(+/-)G(+/-)), C-s(TG(+)G(-)), C-1(G(+/-)TT), and C-3(G(+/-)G(+/-)G(+/-)) structures, given in order of increasing energy. Computations of the transition state structures connecting the C-s(TG(+)G(-)) and C-1(G(+/-)TT) conformers to the global minimum C-1-(TG(+/-)G(+/-)) structure were also carried out. The barriers for the interconversion of C-s(TG(+)G(-)) and C-1(G(+/-)TT) to the ground state C-1(TG(+/-)G(+/-)) conformer were 0.2 and 0.6 kcal/mol, respectively. Comparison of conformational preferences of TMPhite with the related carbon compound, trimethoxymethane, and the organic phosphate, trimethyl phosphate, was also made using natural bond orbital analysis.