Fluorescently labeled polystyrene (PS) tracer molecules are enclosed in a semidilute, entangled solution of PS matrix polymers. The latter carry photo-cross-linkable groups such that the matrix can be successively cross-linked by UV-irradiation. The diffusion coefficient of the labeled tracer chains is measured via FRAP at every stage of the cross-linking process during the transition from sol to gel. In the sol state, significant deviations from the reptation predictions appear unless the matrix chains are considerably longer than the tracer chains. Cross-linking of the matrix chains results in a more or less pronounced decrease of the tracer diffusion coefficient, occurring gradually with rising conversion of the cross-linking reaction, while the observed dependencies of the tracer diffusion coefficient on tracer molecular weight and on matrix concentration converge to what is predicted by reptation theory.