Input-output displacement equations of degree 16 are derived for spatial six-link RRRRCR and RRCRRR2 mechanisms. The derivation of these equations provides the solution to one of the most formidable problems in the theory of analysis of spatial mechanisms. The subsequent displacement analysis is derived by extending Be´zout’s reduction (1764) of the degree of a pair of equations in a single unknown, to equations with two unknowns. Numerical results were verified using a physical model. The input-output equations contain as special cases eighth-degree polynomials for spatial six-link 4R-P-C slider-crank mechanisms.