Fouling of heat exchanger tube surfaces by fireside deposits in recovery boilers greatly reduces the boiler thermal efficiency. A general belief is that fouling may be aggravated by the high SO2 concentration in the flue gas. However, how SO2 affects deposit buildup and removal is not well understood. This work investigated the effect of SO2 on the adhesion strength of deposits and its implications on boiler fouling. Deposits were produced by burning dried black liquor particles in an Entrained Flow Reactor (EFR) at different SO2 concentrations. The deposit adhesion strength was determined by measuring the minimum peak impact pressure of an air jet required to remove deposits from the probe surface. The results showed that, increasing the SO2 concentration in the flue gas did not affect deposit buildup appreciably, but made deposits easier to be removed. Results further show that SO2 did not facilitate the sintering of deposits.