The second-order nonlinear optical response of amine and phenol/ether derivatives (1-8) has been evaluated using experimental and theoretical techniques. Electric-field-induced second-harmonic generation (EFISH) measurements establish that N-phenyl substitution of 4-nitroaniline (1) produces a greater increase in molecular hyperpolarizability than N-methyl substitution. In contrast, O-phenyl substitution of 4-nitrophenol (6) produces a smaller increase in hyperpolarizability than O-methyl substitution. Neither the enhancement of hyperpolarizability upon N-phenyl substitution nor the differential substituent effect is anticipated on the basis of qualitative arguments. Careful theoretical analysis using semiempirical sum-over-states and finite field calculations provide explanations for both observed effects.