This thesis describes the synthesis and characterisation of magnetic oxalato-salts of general formula AMIIMIII(C2O4)3, where A is a monovalent organic cation and MII and MIII are transition metals. Compounds of this type crystallise as honeycomb layers of metal-oxalate sheets interleaved by layers of the cation and may be considered quasi two-dimensional molecular-based magnets. The compounds considered are derived from FeIIFeIII and MnIIFeIII with the cations tetraphenylphosphonium, tetrabutylammonium and alkyltriphenylphosphonium, where the alkyl chain ranges from propyl to heptyl inclusive. Structural characterisation by powder X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) is presented; lattice parameters at room temperature have been determined from XRD and the temperature-dependence of the local structure around the metal ions has been investigated by EXAFS on (n-C4H9)4NFeFe(C2O4)3, (n-C4H9)4NMnFe(C2O4)3, PPh4FeFe(C2O4)3 and PPh4MnFe(C2O4)3 (PPh4 = tetraphenylphosphonium). Bulk magnetic behaviour has been studied by DC magnetometry. Magnetic data in the paramagnetic regime have been fitted to the Curie-Weiss law enabling Curie and Weiss constants to be determined. Variation of the magnetic behaviour by changing the organic cation and by dilution of the magnetic centers with the diamagnetic ions ZnII and GaIII has been investigated. The local magnetic properties of (n-C4H9)4NFeFe(C2O4)3 and PPh4MnFe(C2O4)3 have been probed by muon spin relaxation. The magnetic structure of deuterated examples of PPh4MnFe(C2O4)3 and PPh4FeFe(C2O4)3 has been studied by polarised neutron diffraction. The temperature-dependence of the two-dimensional magnetic correlation length has been determined by fitting these data to a Warren peak shape function.