Microbial biosensors which have genes forbioluminescence coupled to genes that controlhydrocarbon degradation pathways can be used asreporters on the specificity of regulation of thosepathways. Structure-activity relationships can be usedto discover what governs that specificity, and canalso be used to separate compounds into differentgroups depending on mode of action. Published data forfour different bioluminescent biosensors, reporting ontoluene (two separate biosensors), isopropylbenzene,and octane, were analyzed to developstructure-activity relationships between biologicalresponse and physical/chemical properties.Good QSARs (quantitative structure-activityrelationships) were developed for three out of thefour biosensors, with between 88 and 100 per cent ofthe variance explained. Parameters found to beimportant in controlling regulator specificity werehydrophobicity, lowest unoccupied molecular orbitalenergies, and molar volume. For one of the biosensors,it was possible to show that the biological responseto chemicals tested fell into three separate classes(non-hydrocarbons, aliphatic hydrocarbons, andaromatic hydrocarbons). A statistically significantQSAR based on hydrophobicity was developed for thefourth biosensor, but was poor in comparison to theother three (44 per cent variance explained).