This paper, first in a series, examines the effect of thin liquid films on the behaviour of gas-solid fluidized beds. The beds studied represent a limiting case of the broader area of three-phase fluidization, with this regime having potential applications in a number of areas, not least biotechnology. A number of non-volatile silicone oils ranging in viscosity from 0.0196 to 1.04 Ns/m(2) have been examined on glass ballotini ranging in size from 2 to 5 mm in a conventional gas-solid fluidized bed. Oil loadings of up to 3% (wet basis) have been trialed. Liquid addition in this particle mass range tends to initially increase the minimum fluidization velocity above that of the non-wetted bed. However, the fluidization point falls below the dry bed at higher oil loadings. Increased viscosity of the liquid has only a small impact on the minimum fluidization velocity, and a stronger influence on the retardation of bed motion. The results obtained have been explained by a combination of static and dynamic bridge forces.