A mathematical analysis is presented for the unsteady conduction of heat through a growing fouling layer on a planar surface. The aim is to identify when the assumption of a steady state linear temperature profile in the layer, commonly employed both industrially and academically to obtain an estimate of the fouling layer thickness, is not valid. Deviations from the linear profile arise from the volumetric heat capacity of the fouling layer being finite and non-zero, causing thermal inertia to play an important role in this system. The magnitude of the deviations depends upon the numerical values of the thermal diffusivity, the growth rate of the fouling layer, the heat transfer coefficient and the thickness of the fouling layer. A series of criteria for when large errors in the estimated fouling layer thickness (greater than 10%) can arise are presented, for combinations of boundary conditions where the analysis can be applied. A case study of a typical gas-side particulate fouling system shows that estimating the thickness using the linear profile assumption will lead to a 10% error in the estimated fouling layer thickness. (C) 2012 Elsevier Ltd. All rights reserved.