Densities of gamma-butyrolactone and N-methyl-2-pyrrolidone (NMP) have been measured with a computer-controlled high-temperature, high-pressure vibrating tube densimeter system (DMA-HDT) in the liquid state. The uncertainty in density measurement was estimated to be less than +/-0.2 kg(.)m(-3). The densities were measured for temperatures from 273 K to 473 K and pressures from 0.3 MPa up to 40 MPa, whereby a density range between (947 and 1168) kg(.)m(-3) for gamma-butyrolactone (353 data points) and between (865 and 1068) kg(.)m(-3) for NMP (370 data points) was covered. The experimental data were correlated with the three-dimensional density correlation system (TRIDEN) and compared with published data.