Inertial impactors are devices that consist of one or more particle-laden jets impinging onto a surface. Many modern impactors consist of impaction stages that have hundreds of closely spaced jets that, under certain conditions, can influence each other. The objectives of this study were to experimentally quantify the effect of jet interactions on particle collection efficiency, and to determine the underlying collectionmechanism. From this testing, three types of secondary deposits were found, defined as midline, nozzle, and halo. Themagnitude of these deposits varied widely depending on nozzle spacing, Reynolds number, and particle Stokes number. It was found that midline deposition occurs on a line halfway between adjacent nozzles on the impaction plate, and contains up to 8% of particles entering the nozzle. Midline deposition occurs due to a recirculation zone at the intersection of the two jets. Nozzle plate deposition was found to be minimal, comprising less than 3% of particles entering the nozzle. Nozzle plate deposition occurs due to a secondary recirculation zone created against the backside of the nozzle plate. Halo deposition was found to be the largest source of secondary deposition at lowReynolds numbers with up to70%of particles entering the nozzle being deposited at this location. These deposits occur due to particle sedimentation. In general, secondary particle depositions cause the collection efficiency curve to be less sharp, with a shift to lower particle Stokes numbers. Application of these results should drive better impactor design with sharper collection efficiency curves.