An optically accessible internal-mixing air-assisted atomizer has been developed and examined by measurements with a Phase Doppler Analyzer (PDA). The optical access allowed visualization of the flow conditions inside the nozzle by high speed photography. The influence of the outlet port length and several other geometry parameters were investigated within a load range of 10 ≤ M1iq ≤ 60 kg/h and 4 ≤ Mair ≤ 18 kg/h. It was found that the dominant geometric parameter influencing atomization quality is the length of the outlet port L. For short outlet ports energy transfer from the air to the liquid phase is unsatisfactory. Higher values of L provide a longer residence time in the outlet port where relative velocities between air and the liquid phase are high. This leads to improved atomization until an annular film is formed on the wall surface by deposition of ligaments and large droplets. The film leaves the nozzle without being atomized sufficiently which leads to large droplets at the spray boundary. An optimum is reached for L/D = 2 when the entire cross-section of the exit of the outlet port is filled with the two-phase flow. The other geometrical parameters exert an important influence on the preconditioning of the liquid jet and must be considered for the optimization of nozzle design too.