The M{sup 4+}-containing K{sub 2}NiF{sub 4}-type phases La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Fe{sub 0.5}O{sub 4} and La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Mn{sub 0.5}O{sub 4} have been synthesized by a sol-gel procedure and characterized by X-ray powder diffraction, thermal analysis, neutron powder diffraction and Moessbauer spectroscopy. Oxide ion vacancies are created in these materials via reduction of M{sup 4+} to M{sup 3+} and of Co{sup 3+} to Co{sup 2+}. The vacancies are confined to the equatorial planes of the K{sub 2}NiF{sub 4}-type structure. A partial reduction of Mn{sup 3+} to Mn{sup 2+} also occurs to achieve the oxygen stoichiometry in La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Mn{sub 0.5}O{sub 3.6}. La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Fe{sub 0.5}O{sub 3.65} contains Co{sup 2+} and Fe{sup 3+} ions which interact antiferromagnetically and result in noncollinear magnetic order consistent with the tetragonal symmetry. Competing ferromagnetic and antiferromagnetic interactions in La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Fe{sub 0.5}O{sub 4}, La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Mn{sub 0.5}O{sub 4} and La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Mn{sub 0.5}O{sub 3.6} induce spin glass properties in these phases. - Graphical abstract: La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Fe{sub 0.5}O{sub 4} and La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Mn{sub 0.5}O{sub 4} have been synthesized by a sol-gel procedure and characterized by diffraction techniques, thermal analysis and Moessbauer spectroscopy. Oxide ion vacancies, created via reduction, are located in the equatorial planes of the K{sub 2}NiF{sub 4}-type structure. La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Fe{sub 0.5}O{sub 3.65} shows antiferromagnetic, noncollinear magnetic order, whereas competing ferromagnetic and antiferromagnetic interactions in La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Fe{sub 0.5}O{sub 4}, La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Mn{sub 0.5}O{sub 4} and La{sub 0.8}Sr{sub 1.2}Co{sub 0.5}Mn{sub 0.5}O{sub 3.6} induce spin glass behaviour.