The pressure-volume-temperature (PVT) relationships for trimethoxymethylsilane (MTMS, CH3(CH3O)(3)Si) and tetramethyl orthosilicate (TMOS, (CH3O)(4)Si) were measured at temperatures ranging from 303.2 to 363.2 K and pressures up to 160 MPa. Density measurements were carried out with an experimental apparatus based on the variable volume method. The standard uncertainties u(T) = 0.17 K and u(P) = 0.14 MPa, and the relative combined expanded uncertainty U-r (0.95 level of confidence) were U-r(rho) = (0.0016 and 0.0022) for MTMS and TMOS, respectively. The obtained PVT relationships were correlated using the Tait equation and the Sanchez-Lacombe equation of state to determine the characteristic parameters.