Ion-size recognition of three trivalent metal ions (Fe(III), Al(III), Cr(III)) with 3-phenyl-4-acyl-5-isoxazolones derivatives has been studied by the liquid-liquid extraction method. The diffusion of the corresponding complexes with Fe(III) in supported liquid membrane (SLM) system has been evaluated. It was found that the selectivity improves as the 4-acyl groups become bulkier. This improvement could be attributed to the narrowed distance between the two donating oxygens ("bite size"). The comparison of transport experiments of Fe(III) ions by the three acylisoxazolones having different bite size and acidity has showed best results with 3-phenyl-4-benzoyl-5-isoxazolone.