Percolation effects are studied in phase change materials used for optical and electrical nonvolatile memory applications. Simultaneous measurements of the electrical resistivity and optical reflectivity allow experimental assessment of the percolation behavior associated with the mixed amorphous and crystalline phases in this class of chalcogenide materials. Dynamic analysis of the isothermal amorphous to crystalline phase transition in as-deposited amorphous Ge2Sb2Te5 and Sn doped Ge2Sb2Te5 thin films are performed. A significantly earlier onset and saturation for the change of electrical resistivity in comparison to the optically measured phase transition is observed. This behavior reflects a significant influence of percolation on the electrical properties of this class of materials, leading to a nonlinear relationship between crystallization degree and electrical resistivity. This nonlinearity is associated with the formation of highly conducting crystalline paths in a lower conducting amorphous matrix. The influence of percolation on optical properties is found to be negligible. The experimental results are quantitatively compared to Wiener bounds and Bruggeman percolation models. Substantial differences in the crystallization behavior of Ge2Sb2Te5 and Sn doped Ge2Sb2Te5 are observed: while heterogeneous nucleation induced by an interfacial layer resulting in layer-by-layer growth is observed for Ge2Sb2Te5, homogeneou- s nucleation and formation of prism shaped crystallites dominate in Sn doped Ge2Sb2Te5. The presented data represent the experimental analysis of percolation for the electrical resistivity in this class of materials, indicating the relevance of this effect for the adequate modeling and systematic optimization of future phase change random access memories and cognitive devices.