Scanning tunneling microscopy was used to make the first molecular scale measurements of the temperature dependence of composition of an adlayer at the solution-solid interface. We conclusively demonstrate that metal porphyrins adsorb very strongly on Au(111) at the solution solid interface such that the monolayer composition is entirely kinetically controlled below about 100 degrees C. The barrier for desorption is so great in fact that a temperature of 135 degrees C is required to induce desorption over a period of hours. Moreover, cobalt(11) octaethylporphyrin (CoOEP) and NiOEP desorb at different rates from different sites on the surface. We have measured the rate constant for desorption of CoOEP into phenyloctane to be 6.7 x 10(-5)/s at 135 degrees C. On the basis of these measurements, an upper bound can be set for the desorption rate of NiOEP into phenyloctane as 6.7 X 10(-4)/s at 135 degrees C. For solutions of the order of 100 mu M in NiOEP or CoOEP, a dense monolayer is formed within seconds, and the adsorption rate constants fall within 40% of each other. The structures of NiOEP and CoOEP monolayers are essentially identical, and the molecular spacing for both can be described by A = 1.42 +/- 0.02 nm, B = 1.32 +/- 0.02 nm, and alpha = 570 degrees +/- 2 degrees. The solubility of CoOEP and NiOEP in phenyloctane at room temperature was measured to be 0.228 and 0.319 g/L, respectively.