A photoexcited porphyrin system has been found to be an efficient catalyst for D-limonene biotransformation. The catalyst showed high selectivity in hydroxylation of D-limonene. The best catalyst for limonene biotransformation was 5,10,15,20-Tetraphenylporphyrin (H2TPP), which gave the highest accumulation of carvone and an unknown product with a verbenone-like mass spectrum. The highest conversion yield of these products was favoured at 1: 2 molar ratio of H2TPP to limonene. Some factors affecting the biotransformation yield were also investigated. Maximal yield of carvone was obtained in the medium containing 90% of the substrate, within the period of 18-36 h. Mechanism involved in limonene biotransformation catalysed by H2TPP is also discussed.