To study their reliability as primary standards in the quantitative EPR spectroscopy, a large series of pure paramagnetic compounds with known spin concentrations, whose spectra vary considerably in intensity, shape, structure and overall width are compared. The paramagnetic species examined as pure solid compounds and solutions, were free radicals (DPPH and TEMPO), vanadyl and Cul l ions (S = 1/2), Cr" (S = 3/2) and Mu ll (S = 5/2) ions. The quantitative EPR findings suggest that all the S = 1/2 paramagnetic compounds investigated and MnSO4 • H2O (S = 5/2) are reliable primary standards. By contrast, none of the pure Cr" compounds proved useful as primary standards because of their large fine-structure terms or high Néel temperature that invalidated the simple Curie law. Application of quantitative EPR in the study of dilute MgO-MnO and a-Al203 Cr203 solid solutions, focussing on the circumstances making paramagnetic species undetectable, is reported. In MgOMnO solid solutions of high surface area, detection problems arising from the variation of local site symmetry can be circumvented and almost all Mn 2 are detected only by reducing the surface area. In concentrated a-Al2O3-Cr203 solid solutions, magnetic interactions lead to paramagnetic species being undetectable.