The Green Fluorescent Protein (GFP) of Aequorea victoria has shown to be an effective tool for monitoring gene expression or protein localisation in a variety of eucaryotic and bacterial organisms. In this diploma work the application of a GFP-tagged whole cell living biosensor (WCLB) for the assessment of bioavailable octane concentrations will be described. The biosensor strain was based on Escherichia coli DH5a. This strain carried a plasmid (pJAMA30) (17), which contained the regulatory genes alkS, alkT and their promoter PalkST from Pseudomonas oleovorans combined with a transcriptional fusion between the inducible promoter PalkB from P.oleovorans and a promoterless mutant gfp gene (S65T/F64L). The E.coli DH5a(pJAMA30) octane biosensor was used to determine diffusion of octane in a flow cell by attaching the sensor bacteria to the glass and measuring the expression of GFP in individual bacterial cells along a distance. The GFP fluorescence intensity of single bioreporter cells was recorded by epifluorescence microscopy and digital imaging. The intensities of GFP fluorescence were calibrated in standardised assays with known concentrations of octane. GFP intensities increased almost linearly upon induction with octane concentrations between 0 and 0.1 mM, 0.5 mM, or 1 mM, depending on the system. At higher octane concentrations no further increase of GFP expression levels was observed. Maximal GFP fluorescence intensities were obtained after an induction period of 2.5 h. Diffusion of octane was determined when dissolving from (i) a pure octane droplet, (ii) a droplet of an alkane mixture (heptamethylnonane/octane), and from a HMN/octane droplet surrounded (iii) by an air bubble or (iv) by an argon/CO2 bubble. Under all conditions induction of the GFP expression in the alkane-GFP biosensor was observed, indicating that octane reached the cells. Model calculations on diffusion rates predicted similar outcome of our observations.