The design and construction of a phosphorimeter especially suited for investigating the effect of a magnetic field on delayed luminescence processes is described. Although it is designed for the light levels and time domains usually pertinent to fluid-solution delayed fluorescence studies, it can be easily adapted to phosphorescence work. In order to avoid the influence of the applied field on the excitation and detection elements, these components are removed from the field and are heavily shielded. A medium-pressure mercury arc is used as an excitation source, and either frontal or right-angular illumination of the sample can be employed. Filters in both the excitation and emission beams provide the means of wavelength selection. The use of a single, rotating, slotted disk to time both beams gives the instrument a compact design and enhances its physical stability. Tests of instrument performance show that the phosphorimeter is useful for measurements at fields up to 8000 G.