The results of a field and laboratory investigation of unconsolidated sediments contaminated by petroleum hydrocarbons and undergoing natural biodegradation are presented. Fundamental to geophysical investigations of hydrocarbon impacted sediments is the assessment of how microbial degradational processes affect their geoelectrical response. Therefore, the primary goal of this study was to understand how microbially mediated processes in hydrocarbon impacted sediments influence the geoelectrical response of this impacted zone. The field and laboratory results showed higher bulk conductivity in sediments impacted by petroleum hydrocarbons. The impacted sediments also showed increased populations of alkane degrading microbes and elevated dissolved cations. The elevated cations in the contaminated sediments relative to uncontaminated sediments suggest enhanced mineral dissolution related to the microbial degradation of the hydrocarbon. Both the laboratory and field data showed the highest bulk conductivities occurring within zones impacted with the free-phase and residual phase hydrocarbon and not within the water saturated zone.