Attempts to understand and control wetting phenomena at organic surfaces have been hampered by a lack of molecular understanding of the interfaces, in particular, the extent; of interpenetration and reconstruction of the adjoining phases. Through molecular dynamics simulations we show that subtle details of interfacial reconstruction can dramatically affect the stability of interfaces between a wetting Liquid and a self-assembled monolayer(SAM). These results strikingly rationalize experiments which show a sharp transition in contact angles with small variations in chain length of the SAM. This finding provides an initial basis for the quantitative description of wetting phenomena, e.g., contact angle measurements, and offers significant progress toward a general molecular-based understanding of more complex interfacial phenomena observed with macro-and biomolecular systems.