A series of 16 specimens of concrete web panels 23.6 x 23.6 x 4.9 in. (600 x 600 x 125 mm) was tested in compression to investigate the effect of the presence of various types of post-tensioning ducts on the strength of the shear-induced compression struts. Most panels were cast in the laboratory, but some were extracted from an actual bridge girder, which had been previously loaded, allowing investigation of the effect of web cracking on the ultimate strength. The presence of a duct in a web, whether injected or not, decreases the compressive strength of the panel. This effect is most pronounced for non-injected ducts, but is also much larger for injected plastic ducts than for injected steel ducts. The effect of web cracking further decreases the strength of web panels containing post-tensioning ducts, and this effect can be estimated using classical strength reduction formulas.