Air-side fouling of compact heat exchangers (automobile radiators) has been a preoccupation for industrial users of off-road vehicles for many years. Until recently, most studies examined the effects of small sized particles in the 1-100µm diameter range. The present work concentrates on fouling of the inter-fin space (IFS) by binary mixes of larger particles (up to some mm in length). Each mix comprises 95% by weight of "smaller" particles and 5% "larger" particles. The threshold size is the critical value of 0.63 times the maximum IFS. Two exchangers (A & B) are studied experimentally with maximum IFS(A) being 3mm with particle sizes up to 4mm in length and exchanger B having corresponding values of 1.38mm and 1.6mm respectively. Pressure drop and the proportion and dispersion of particles that contribute to fouling are measured as a function of air speed (up to 5m/s or 18km/h, typical of off-road vehicles) and particle size mix for isothermal and non-isothermal conditions. These are complemented by visualisation. The results show that the foulant is not simply the cumulative effect of both particle sizes. There is a distinct interaction between them leading to a greater number of particles being blocked on the exchanger. The physics behind this are discussed in detail. The importance of the critical size particles in the augmentation of fouling is clearly demonstrated and explained. It is possible to define an equivalent particle size for any binary mixture by comparison with the pressure drops for mono-disperse particle sizes leading to a unique curve for the fouling factor reduced by exchanger surface and particle mass as a function of non-dimensional particle size. Heat transfer measurements (with water temperature at 60°C or 70°C at exchanger entry) in a wind tunnel with closed and open sides show the dangers of the former leading to erroneous conclusions.