The invention relates to a process to produce HCFC-244bb from HCFO-1233xf wherein, in one embodiment, one or more co-feed species having a normal boiling point of between about −80° C. to about 0° C., such as HFC-245cb, is added to the reaction at a pressure of at least about 100 psig; and in another embodiment it is added to maintain a mole ratio of HFC-245cb to HCFO-1233xf of between about 0.005:1 to about 1:1. The HFC-245cb may be added as recycled by-product of the reaction and/or added as fresh feed. The HFC-245cb provides elevated pressures to the reaction thereby facilitating reactor operation, mixing and HCFC-244bb product removal. Other co-feed species are also disclosed.