Measurement of diesel and spark ignition engine particulate emissions is of wide interest due to current research demonstrating that inhalation of nanoparticles may cause serious health problems. Both experimental and computational methods were used to investigate fluid and particle flows through a miniature dilution tunnel similar to those commonly used to sample particulate emissions from engines to help explain observed varabilities and bias, and to improve the repeatability of results. Laser Doppler Velocimetry, flow visualization, and a commercial CFD code were used to measure the flow field inside the apparatus. Slugs of NOx calibration gas and a high speed NOx concentration meter were used to measure mean velocities through the apparatus. An artificial aerosol generator was used in conjunction with a Scanning Mobility Particle Sizer (SMPS) to determine how tunnel geometry affects particle size distributions. These results were compared to a Monte Carlo type numerical simulation, accounting for Brownian motion and fluctuating turbulent velocities.