The magnetic properties of the ferrite Mn0.25Zn0.75Fe2O4 have been studied by Mossbauer spectroscopy and magnetisation measurements. Mossbauer spectra, obtained at low temperatures and large applied magnetic fields, showed that almost all Fe3+ ions are in octahedral (B) sites and only about 2% of the Fe3+ ions are in tetrahedral (A) sites. At 5 K about 16% of the B-site Fe3+ ions have spins parallel to the A-site magnetisation, but when the temperature is raised to about 40 K, these spins are reversed. Some other B-site Fe3+ ions show a large spin canting at 5 K and these ions are significantly affected by transverse relaxation at relatively low temperatures. A third group of 13-site ions has negligible canting at low temperatures and these ions are only slightly affected by relaxation below 60 K. At low temperatures the material can be described as a cluster spin glass. Above 60 K part of the sample breaks up into superparamagnetic clusters whereas other parts remain ferrimagnetic.