A method to determine wall thinning defect depths using the peak contrast slope method based on the transmission mode of pulsed IR thermography is presented. An analytical one-dimensional model of transient heat diffusion is used to deduce the proportionality factor between the time at peak contrast and the defect depth for a wide range of defect depths. The method has been corroborated with finite element model-based simulations. Experimental validation was carried out for four defect depths in steel. © 2010 Taylor & Francis.