The reaction path for the synthesis of LaFeAsO and LaFeAsO_1−xF_y by solid state reaction was studied by in situ high temperature x-ray diffraction technique and differential thermal analysis in the temperature interval 100 °C ⩽ T ⩽ 1150 °C. Starting with LaAs, Fe₂O₃, Fe, and LaF₃ as precursors, the results show that the synthesis is characterized by three temperature intervals: (1) Below 500 °C the sequential reduction of Fe₂O₃ and Fe₃O₄takes place through the oxidization of LaAs. Below 400 °C, Fe₂O₃ is reduced to Fe₃O₄ by LaAs and then at 400 °C<T<500 °C Fe₃O₄ is further reduced to Fe. (2) In the temperature interval 500 °C<T<800 °C, multiple intermediate reactions take place resulting in the formation of FeAs and La₂O₃. (3) The formation of LaFeAsO based phase could be unambiguously resolved above 800 °C. For both LaFeAsO and LaFeAsO_1−xF_y, FeAs is a primary impurity at high temperatures that melts at ∼ 1040 °C. Possible reaction pathways and the difference between F-free and F-doped samples are discussed.