We study the boundary value problem − div(log(1 + |∇u| q )|∇u| p−2 ∇u) = f (u) in Ω, u = 0 on ∂Ω, where Ω is a bounded domain in R N with smooth boundary. We distinguish the cases where either f (u) = −λ|u| p−2 u + |u| r−2 u or f (u) = λ|u| p−2 u − |u| r−2 u, with p, q > 1, p + q < min{N, r}, and r < (Np − N + p)/(N − p). In the first case we show the existence of infinitely many weak solutions for any λ > 0. In the second case we prove the existence of a nontrivial weak solution if λ is sufficiently large. Our approach relies on adequate variational methods in Orlicz-Sobolev spaces.