Calorimetric heats of adsorption and sticking probabilities are reported for NO and CO on both the reconstructed hex and the unreconstructed (1 X 1) surfaces of Pt{100} by single crystal adsorption calorimetry (SCAC), at room temperature. The hex surface reverts to the (1 X 1 ) structure during adsorption of both gases, as previously reported. The initial heat of adsorption on the (1 X 1) surface is 215 kJ/mol for CO and 200 kJ/mol for NO. Adsorbate-adsorbate interactions determine not only the dependence of the heat of adsorption on coverage but also the formation of different ordered structures. A model is suggested to explain the observed dependence of the differential heat on coverage and the LEED patterns, and a Monte Carlo simulation is performed to derive the corresponding differential heat, thus allowing estimates to be made of the magnitude of adsorbate-adsorbate interactions. For CO adsorption, the critical contribution is the pairwise interaction energy epsilon(d) between molecules in nnn sites while for NO triplet formation is suggested with significant repulsive interaction between molecules in the same tripler (epsilon(t)) and an even stronger repulsion between triplet pairs (epsilon(tt)). NO-NO repulsive interactions (epsilon(t) = 20 kJ/mol, epsilon(tt) = 80 kJ/mol) are considerably stronger than GO-CO interactions (epsilon(d) = 5 kT/mol); thus, at half monolayer coverage CO gives rise to a c(2 X 2) pattern while NO gives a c(2 X 4) pattern. Moreover, with CO the coverage can be increased to 0.75 ML, with the formation of compressed structures, while for NO the saturation coverage is just 0.5 ML. The differential heat on the hex surface is also discussed showing the possible role of adsorption at defect sites in the energetics of the system. The surface energy difference between the clean (1 X 1) and hex surfaces is obtained as 20 kJ(mol Pt-s)(-1) by comparing the integral heats of adsorption of CO on both surfaces at theta = 0.5, when the final states of the two systems are identical.