An algebraic treatment is developed for the effect of a plane substrate with isotropic relative permittivity on the linear optical response of layers of molecules treated as sets of point submolecules so as to incorporate molecular elongation and orientation. Results are modified from those for free-standing films by planewise sums describing the field at a dipole due to its image in the substrate. Numerical calculations of refractive indices are performed for a monolayer of molecules initially packed in a tetragonal structure normal to the substrate and then tilted to yield monoclinic structures. The head-group polarizability is varied relative to that of the other submolecules, as is the spacing between the layer and the substrate. Substrate effects are largest when a very polarizable head group of a tilted molecule is adjacent and very close to the substrate; in extreme cases the refractive indices diverge. The results support the idea that first-layer effects on optical properties could be dielectric as well as structural in origin.