A calorimetric investigation of the monolayers (MLs) of tetramethylsilane (TMS) adsorbed on graphite demonstrates two remarkable features: (i) occurrence of an orientational order-disorder phase transition in the 2-D solid (at 107 K at the coverage theta < 1 and at 138 K at 1 < theta < 2) accompanied by a large entropy change (ranging from R In 8 to R In 3) depending on the coverage, and (ii) possible formation of a fluid bilayer around 220 K, above the 2-D critical temperature. A high-resolution incoherent neutron scattering experiment reveals a considerable motional disorder in the disordered 2-D solid.