Void formation during vacuum bag curing of epoxy him adhesives arises from extraneous volatile products. Potential sources of volatiles were found to include organic solvents and/or water remaining in the adhesive from manufacture, water picked up by the adhesive prior to curing, and water retained in various forms on the prepared adherend surface. Using a novel FTIR spectroscopy technique, water and methyl ethyl ketone (MEK) were identified as the principal volatiles formed during curing, and quantitative estimations showed batch-to-batch variations and the tendency of uncured adhesive to rapidly absorb atmospheric water. An estimation of void content was obtained from video images of fractured, adhesively bonded joints using a computerized pixel-counting technique. The conclusions drawn from tensile shear, peel, and wedge durability tests suggest that a void content in excess of 25% lowers T-peel and honeycomb peel strengths and may affect bond durability. Void content can be minimized using an in situ heat/vacuum outgassing treatment.