The effect of the chain length and temperature on ordering of alkylphosphonate, (CnH2n+1)PO3H2, n = 8-18, monolayers on ZrO2 was characterized by solid-state C-13 NMR. Alkylphosphonate monolayers with chain lengths of n < 16 are disordered at room temperature. C-18 monolayers of thiols and phosphonates both show reversible chain melting at similar to 50 degrees C. This chain length dependence, as well as variable-temperature spectra, demonstrates that the thermal stabilities of conformational order in phosphonate and thiol monolayers are similar. In general, the initial free volume in a self-assembled monolayer on a rough surface is large enough to allow reversible thermal disordering although the headgroup remains rigid.