The pressure versus area curves for a spread myristic acid monolayer are markedly affected by the compression velocities as becomes clear from the published results of K. Motomura, A. Shibata, M. Nakamura, and R. Matuura (J. Colloid Interface Sci. 29, 623, 1969). This is due to the dissolution of spread molecules into the bulk solution. From these apparent Pi-A curves the thermodynamic Pi-A curve (without a phase transition), where no diffusion is present, can be constructed in three different ways. A qualitative explanation for the different diffusion relaxation times obtained by each approach is given.