The pairing of quasiparticles in a CuO2 plane is studied within a spin polaron formulation of the t-t ′ -J model. Our numerical solution of the Eliashberg equations unambiguously shows d-wave pairing between spin polarons on different sublattices mediated by the exchange of spin-fluctuations, and a strong doping dependence of the quasiparticle bandwidth. The transition temperature Tc is an increasing function of J/t and crosses a maximum at an optimal doping concentration δopt. For the t-J model with J/t = 0.4 we obtain Tc ≃ 0.013t at δopt ≃ 0.2.