Stable drop jettability is mandatory for a successful, technical scale inkjet printing, and accordingly, this aspect has attracted much attention in fundamental and applied research. Previous studies were mainly focused on Newtonian fluids or polymer solutions. Here, we have investigated the drop jetting for zinc oxide (ZnO) particulate suspensions. Generally, the inverse Ohnesorge number Z = Oh(-1), which relates viscous forces to inertia and surface tension, is sufficient to predict the jettability of single phase fluids. For the inkjet printer setup used here, jetting was possible for Newtonian fluids with 2.5 < Z < 26, but in the identical Z-range, nonjetting and nozzle clogging occurred for certain suspensions. A so-called ring-slit device, which allows for simultaneous formation and detection of aggregates in strongly converging flow fields, and single particle detecting techniques, which allow for an accurate determination of the number and size of micrometer-sized aggregates in suspensions of nanoparticles, were used to study this phenomenon. Nozzle clogging is induced by heterocoagulation of micrometer-sized aggregates and ZnO nanoparticles in the elongational flow field at the nozzle exit. Clogging may occur even if the size of these aggregates is well below the nozzle diameter and their concentration is on the order of only a few hundred parts per million (ppm). Accordingly, increased colloidal stability of nanoparticles and reduced aggregate concentration result in better drop jettability. Also, a nozzle design resulting in a shorter exposure time of the ink to elongational flow and an increased flow velocity helps to avoid nozzle clogging.