This invention relates to a process for producing 1,1,1,2-tetrafluoroethane (HFC-134a). The process reacts, 1,1,1-trifluoro-2-chloroethane (HCFC-133a) and hydrogen fluoride in a first reactor. The product resulting from the first reaction step is brought to a second reactor together with trichloroethylene and hydrogen fluoride. The second reaction is conducted at a higher temperature than the first reactor. Optionally, HCl is removed prior to removal of the crude HFC-134a product. Unreacted HCFC-133a, trichloroethylene and hydrogen fluoride may be recycled back to the first reactor.