We discuss the dependence of the population transfer probability in the three-level Lambda-scheme using stimulated Raman adiabatic passage on detuning from the two-photon resonance. For a large decay rate of the intermediate level we found that the two-photon lineshape is close to Gaussian with linewidth inversely proportional to the square root of the decay rate. The shift of the maximum of the two-photon line from exact two-photon resonance has been investigated. (C) 1997 Elsevier Science B.V.