Single crystals of (Mg₉₉.₇₅Mn₀.₂₅)₂P₂O₇ have been investigated by esr through the temperature range from room temperature to 200ºC. One phase transition was observed extended through the temperature range between 59.5ºC to 63ºC where the α-and β phase coexist. Accurate spectra were recorded at 74ºC, 84ºC and 94ºC at the three magnetic axis, and the spin-Hamiltonian parameters were obtained from these data. The phase transition mechanism and the significance of the spin-Hamiltonian parameters in the theory of S-state splitting are discussed in the light of current literature. / Thesis / Master of Science (MSc)