An equation of state for multilayer adsorption is derived. This equation gives the correct thermodynamic behavior at small pressures and at the saturation vapor pressure. The equation of state is analyzed for weakly attractive and strongly attractive surfaces and results are compared with experimental data. In addition, a new relationship is obtained for the exponent d which describes the singularity in the multilayer adsorption isotherm, It is shown that d depends on temperature, saturation vapor pressure, Henry’s coefficient, monolayer capacity, and surface tension.