Processes for thermal conversion of vacuum residues in fluidized or moving beds of solid particles require excellent distribution of feed liquid onto the solid particles and free flow of the solids. Forces between the solid particles will be determined by the changing quantity and properties of the liquid material during reaction. This paper presents a unique apparatus for measuring pull-off forces between particles due to reacting liquid films. In the case of 25-30-mum-thick films of Athabasca vacuum residue at 503 degreesC, the maximum pull-off force occurred after ca. 12 s of reaction and then declined gradually until no force was detected after ca. 24 s of reaction. These data suggest that solid particles coated with vacuum residue would be adhesive for this period of time, while the film of feed material is still liquid and undergoing reaction.