Bacteria communicate chemically through a system called quorum sensing. In this work, microdialysis sampling procedures were optimized to collect quorum sensing molecules produced during in situ biofilm formation directly on the polymeric semipermeable membrane of the microdialysis probe. V. harveyi, a Gram-negative bacterium, was used as the model organism and releases variable chain length acylhomoserine lactones (AHLs) and acyl-oxohomoserine lactones (AOHLs) as signaling molecules during quorum sensing. Eliciting biofilm formation required coating fetal bovine serum onto the poly­(ether sulfone) microdialysis membrane. Dialysates were collected in different experiments either during or after biofilm formation directly on a microdialysis probe. Continuous sampling of C4-AHL, C6-AHL, C8-AHL, C6-OXO-AHL, and C12-OXO-AHL was achieved over a period of up to 4 days. The AHLs and AOHLs in dialysates were concentrated with solid-phase extraction and quantified using LC-MS. Dialysate concentrations obtained for the AOHLs and AHLs ranged between 1 and 100 ppb (ng/mL) and varied between sampling days. This work demonstrates the initial use of microdialysis sampling to collect quorum sensing signaling chemicals during biofilm formation by a Gram-negative bacterial species.