In this paper. (1) We determine the complex-valued solutions of the following variant of Van Vleck's functional equation ∫ Sf(σ (y)xt)dμ (t)− ∫ Sf(xyt)dμ (t)=2f(x)f(y), x, y∈ S, ∫ Sf(σ (y)xt)dμ (t)− ∫ Sf(xyt)dμ (t)=2f(x)f(y), x, y∈ S, where SS is a semigroup, σ σ is an involutive morphism of SS, and μ μ is a complex measure that is linear combinations of Dirac measures (δ zi)i∈ I(δ zi)i∈ I, such that for all i∈ Ii∈ I, zizi is contained in the center of SS. (2) We determine the complex-valued continuous solutions of the following variant of d'Alembert's functional equation ∫ Sf(xty)dυ (t)+∫ Sf(σ (y)tx)dυ (t)=2f(x)f(y), x, y∈ S, ∫ Sf(xty)dυ (t)+∫ Sf(σ (y)tx)dυ (t)=2f(x)f(y), x, y∈ S, where SS is a topological semigroup, σ σ is a continuous involutive automorphism of SS, and υ υ is a complex measure with compact support and which is σ σ-invariant. (3) We prove the superstability theorems of the first functional equation.