Solid phantoms are widely used in radiation therapy for both relative and reference dosimetry. Two water equivalent phantoms, RMI-457 Solid Water and Plastic Water, were evaluated for use in kilovoltage x-ray dosimetry in the energy range from 75 to 300 kVp. Relative and reference dosimetry measurements were performed in the solid phantoms and compared with water. The results indicate that RMI-457 Solid Water could be used for output factor determination for all energies tested and the measurement of percentage depth doses for the 300 kVp x-ray beam, with data agreeing to within 1%, compared to the same measurements in water. For the same criteria, Plastic Water could only be used for output factor determination of the 300 kVp x-ray beam. The superior agreement of the calculated mass-energy absorption coefficients for Solid Water and water, as compared to Plastic Water and water was consistent with the experimental results. Reference dosimetry is not recommended with the solid phantoms for the energies studied due to the lack of published correction factors. It is recommended that any solid phantom be tested by comparison with water in the same manner before being used for the dosimetry of kilovoltage x-ray beams.