Experiments were conducted to study wave occlusion on falling films in a vertical pipe with aqueous solutions of glycerine with viscosities from 0.022 to 1.25 kg/(m.s). Pipe diameters were varied from 0.00635 to 0.015875 m. For each case, the critical liquid flow rate was determined at which the free falling wavy annular flow transforms into slug flow due to wave occlusion. The experimental results are correlated in terms of the liquid Reynolds number at occlusion (Re-LS = 4 q/pi nu D) and two dimensionless groups influencing the transition: the Kapitza number Ka = sigma/[(nu g)(1/3)mu], and the Bond number Bo = rho gD(2)/sigma. The annular-slug transition correlation was Re-LS = 0.062 Ka(4/3)Bo(2/3) and valid in the range 0.1 < Re-LS < 200, 0.2 < Ka < 55 and 7 < Bo < 50. Experimental results were compared to related published results, and the experimental correlation is explained qualitatively.