Replacements occur of halide (X) by oxalate ion (C₂O₄^2–, ox) in the complexes trans-[RhL₄X₂]X [L = pyridine (py), 4-methylpyridine, and 3,5-lutidine] to give [RhL₃X(ox)]. A similar (but catalysed) formation from trans-[Rh(en)₂Cl₂]Y of cis-[Rh(en)₂L]Z (Y = NO₃^– or Cl^–; Z= ClO₄^–, NO₃^–, or S₂O₆^2–; en = ethylenediamine) has been achieved [L = oxalato(2–), malonato(2–), glycinato-O, or L-alaninato-O]. The utility of the oxalato-complexes in reactions (i) is illustrated for (a) L = L′=½en, X = Cl or Br, giving a convenient route into the cis-[RhL₃L′(ox)]ⁿ⁺+ 2HX →[RhL₃L′X₂]^n–+ H₂Ox (i) bis(ethylenediamine)rhodium(III) series, and (b) L = py, L′= Cl, X = Br, or L = py, L′= Br, X = Cl, giving novel mixed halogeno-complexes. Properties of the complexes [RhL₃X(ox)](L = py) are described, with vibrational spectra assigned through use of perdeuteriopyridine. The photosensitivity (originally noted by Poulenc in 1935) is shown to arise from reaction (ii). Some properties of the novel rhodium(I)-containing products of these [Rh(py)₃(ox)X]→[Rh(py)₃X]+ 2CO₂(ii) photoreductions are also described.