The effect of chemical interactions on the transport of caesium in circuits under laminar flow conditions in the presence of boron is studied by analysing deposition profile data from two experiments. The experiments were performed at AEA Technology, Winfrith (U.K.). Predictions of two transport computer codes, RAFT and VICTORIA, are compared with experimental data. Code calculations and experimental data show that in the presence of excess boron volatile caesium hydroxide converts to less-volatile caesium berate, thereby modifying caesium transport. In experiment one, where steam transported caesium hydroxide, both codes accurately predicted thermophoretic deposition and the position of a caesium hydroxide condensation peak. Thermo-dynamic data for caesium hydroxide species are compared and values for the effective caesium hydroxide vapour pressure are suggested. In experiment two, where steam transported boric acid and caesium, both codes underestimated caesium deposition. The discrepancy is partly due to uncertainties in release rates and in wall and gas temperature measurements. Code predictions are in agreement with the experimental observation that caesium hydroxide and boric acid reacted to form caesium berate. Transport codes with fixed speciation can be ＇＇tuned＇＇ to either experiment but not to both simultaneously.