Previous research has established that retrofitting RC (reinforced concrete) columns with FRP (fiber reinforced polymer) jackets is an effective means of providing external confinement to the column cross section. In an effort to further FRP jacketing technology, this study focused on the confinement effectiveness of FRP jackets for rectangular building-type RC columns. Twenty-six fourth-scale short columns were tested to failure in axial compression. Variables investigated include the following: the type of fibers (AFRP, CFRP, or GFRP), the thickness of the jacket, the shape (rectangular or circular) and aspect ratio of the cross section, the amount of longitudinal steel reinforcement, the spacing of the transverse steel reinforcement, and the sharpness of the corners of the rectangular cross sections. Unjacketed columns were included in the testing matrix for reference. The specimens were instrumented to measure axial deformation, as well as the distribution of the hoop strain in the jackets. For square columns, GFRP jackets were observed to increase the ultimate axial strain more effectively than either AFRP or CFRP jackets. For multiple-ply jackets on square columns, GFRP was also found to be the most effective at increasing the ultimate axial normalized stress. Increasing the aspect ratio of the rectangular cross sections resulted in a decrease in confinement effectiveness, as did increasing the sharpness of the corners. The ultimate axial stresses sustained by the jacketed specimens were compared to the predictions of a model from the literature. The agreement between the model and the experimental results varied widely, but on the average the model overestimated the strength of the columns by about 14%.