A new technique based on optical microscopy and particle size spectrometry is here presented to investigate deposition of aerosol particles. Micron-sized monodisperse liquid particles were injected in a horizontal duct flow. The turbulent flow field was recorded using Particle Image Velocimetry. The airborne particle size and number concentration was determined by isokinetic sampling in combination with an Aerodynamic Particle Sizer (APS). The surface deposited particles were counted by means of optical microscopy, which was found to be a robust and simple recording method. The particle deposition velocity was directly determined out of the number of deposited particles detected by the microscope and the airborne particle number concentration recorded by the APS. The present results show good agreement with similar studies and prove the potential of optical microscopy for the detection of wall deposited particles in comparison to the more complex and expensive methods such as fluorescence based wet chemistry or neutron activation techniques. (C) 2013 Elsevier Ltd. All rights reserved.