Dodecane is deposited at submonolayer coverages onto an Au(I 11) surface forming two-dimensional (2-D) islands. The islands sublimate to a 2-D gas at higher substrate temperatures. We observe island formation and subsequent 2-D sublimation between substrate temperatures of 40 and 350 K, using low-energy helium reflectivity. A computer model of the submonolayer islanding process using Monte Carlo simulations shows significant agreement with experimental data and yields an intermolecular potential of 0.10 +/- 0.03 eV (about half that of the bulk substance) and a significantly higher corrugation potential of 0.3 +/- 0.1 eV.