The theoretical equations of state for a Lennard-Jones fluid as determined by a variety of theories (Percus-Yevick, hypernetted-chain, Barker-Henderson, Weeks-Chandler-Andersen, Andrews, and virial expansion) are compared with experimental data for both argon and krypton monolayers. The experimental two-dimensional equation of state for argon is obtained from adsorption isotherms for argon on graphitized carbon black and is reported here over a range of temperatures from 120 to 166 K. The Percus-Yevick-energy equation of state is found to give the best overall agreement with experimental results. © 1979 American Institute of Physics.