The phase transition behavior of spherical supported lipid monolayers (SSMs) of symmetric-chain phosphatidylcholines of different chain length was studied by a combination of differential scanning calorimetry (DSC) and deuterium nuclear magnetic resonance (H-2-NMR). The SSMs are self-assembled lipid monolayers on silica gels, which were functionalized by an alkyl chain layer. We find that the chain melting temperature T-m,T-m of the monolayer differs from the main phase transition temperature T-m,T-b Of a bilayer of the corresponding lipid in a systematic way. The difference temperature, T-m,T-m - T-m,T-b, shows a linear dependence on the lipid chain length for a given surface-attached all-trans alkyl chain, with a zero crossing for the case of when the chain lengths of lipid and attached alkyl match. This behavior suggests that interdigitation between lipid and attached alkyl chains occurs under gel phase conditions. A comparison of H-2-NMR gel phase line shapes of SSMs and of multilamellar vesicles (MLV) at different temperatures further supports the existence of an interdigitation in SSMs. A phenomenological equation is suggested that describes the dependence of T-m,T-m on the lipid chain length for a given all-trans alkyl chain. It is based on the established T-m,T-b dependence on lipid chain length in MLVs but has been modified by a term that accounts for the chain length mismatch between lipid and attached alkyl.