In this study, the formation of guaiacol from vanillin by Alicyclobacillus acidoterrestris in apple juice was investigated. Apple juices spiked with 10 and 100 mg/L of vanillin were inoculated with 1 x 10(3) CFU/mL and 1 x 10(5) CFU/mL spores of A. acidoterrestris and incubated at 25 and 46 degrees C. A. acidoterrestris started to form guaiacol from vanillin when the spore count exceeded the critical level of 1 x 10(4) CFU/mL. Increasing the vanillin concentration also increased the concentration of guaiacol formed, especially at 46 degreesC. The growth of A. acidoterrestris was not favorable at 25 degreesC. Thus, the formation of guaiacol is strongly limited at room temperature.