Trisodium meso-tetra-4-sulfonatophenylporphinerhodium(III) (Rh(SPP)) was found to be a very effective catalyst for cleaving the β-aryl ether bonds of the lignin model compounds guaiacylglycol β-guaiacyl ether (1) and guaiacylglycerol β-guaiacyl ether (11). In both cases very large amounts of guaiacol (2) were formed regardless of whether the reducing sugar glucose was present or not. With the substituted phenylethane model 1 the other major product was acetovanillone (5); with the substituted phenylpropane model 11 considerable amounts of vanillin (4) were formed. The products obtained with the rhodium catalyst were very different to those formed in the Co(SPP) or anthraquinone (AQ) catalyzed reactions. They were, however, similar to those obtained with Fe(TSPc), although the rhodium catalyst was much more active and formed guaiacol at a much faster rate. In addition to these reactions, the rhodium complex also catalyzes the partial demethylation of the 3, 4-dimethoxyphenyl model compound 10. Mechanisms are proposed for these reactions in which the key steps involve single electron transfer between the catalyst and the substrate.