A new vibrating-tube densimeter has been constructed for measuring the relative densities of fluid systems in the elevated temperature and pressure ranges 323 less than or equal to T/K less than or equal to 533 and 0.1 less than or equal to p/MPa less than or equal to 40. The instrument utilizes a light beam in the collection of a signal and dispenses with the usual pick-up and drive wire combinations required in other instruments. The densimeter has been tested by measuring relative densities for aqueous sodium chloride solutions at temperatures from T = 398 K to T = 473 K and pressures from p = 10 MPa to p = 30 MPa. Calculated densities have been compared to those previously published in the literature in order to assess the precision of our new instrument.Also, relative densities for glycine in water have been measured at temperatures from T = 398 K to T = 473 K and pressures from p = 10 MPa to p = 30 MPa over an extended concentration range. Calculated standard state volumes are compared to those which may be calculated from previously published revised Helgeson, Kirkham, and Flowers equations of state.