On-site monitoring of volatile fatty acids (VFAs), such as propionate, is industrially and medically important. The present study developed a VFA biosensing system comprised of two recombinant enzymes, propionate coenzyme A (CoA) transferase (PCT) from Clostridium propionicum and acyl-CoA oxidase from Arabidopsis thaliana. This system produced hydrogen peroxide in the presence of acetyl-CoA, oxygen, and VFA substrates, which could be quantified by colorimetric methods using peroxidase and dye reagents (e.g., p-aminobenzoic acid plus 4-aminoantipyrine or Amplex Red). The use of PCT and acetyl-CoA, rather than acyl-CoA synthetases (ACS) and CoA-SH, obviated a background reaction of dye reagents with CoA-SH and enabled very sensitive detection of VFAs (down to 1 mu M propionate, more than 100-fold more sensitive compared to previously developed ACS biosensors). We demonstrated its utility by measuring propionate concentrations in serum and fermentation samples. Results suggest that our biosensing system is applicable to the detection of propionate in medical and fermentation samples.