The critical parameters of R-404A, a ternary mixture of 44 mass % pentafluoroethane (R-125), 52 mass % 1,1,1-trifluoroethane (R-143a), and 4 mass % 1,1,1,2-tetrafluoroethane (R-134a), were measured by visual observation of the meniscus in an optical cell. The critical pressure, density, and temperature were determined to be (3.726 +/- 0.002) MPa, (490 +/- 3) kg.m(-3), and (345.15 +/- 0.01) K, respectively. Forty single-phase PVT properties have been measured with a constant-volume method along three isochores in a range of densities from 487 kg.m(-3) to 493 kg.m(-3), temperatures from 345 K to 393 K, and pressures from 3.7 MPa to 7.8 MPa. One hundred and nineteen densities of R-404A have been measured with a vibrating-tube densitometer over a density range from 47 kg.m(-3) to 1247 kg.m(-3) along eight isotherms between 263 K and 403 K and at pressures between 1.5 MPa and 15 MPa. An 18-term modified Benedict-Webb-Rubin equation of state for R-404A was developed using the PVT property data.