Preferential flow in the field might be caused by various factors and is difficult to observe in situ. This experimental study was designed to identify the combined effects of air entrapment, surface desaturation (suction head), soil layering, and water repellency (hydrophobicity) of the porous media on unstable preferential flow (or fingering) in the vadose zone. The predictability of unstable flow was studied on the basis of two existing criteria for gravity fingering: (1) a velocity criterion proposed by Hill and Parlange <1972> and (2) a pressure head criterion by Raats <1973> and Philip <1975>. Two-dimensional transparent chambers (60 cm high, 41.5 cm wide, and 2.8 cm thick and 90 cm deep, 74.5 cm wide, and 1.8 cm thick) were used to visualize water infiltration into a water-wettable sand, a water-wettable loam, differently layered sand and loam, and a water-repellent sand. The results suggested that infiltration into the homogeneous sand and a sand-over-loam system, without the effects of air entrapment and surface desaturation, was unconditionally stable. Infiltration in the loam was also stable as observed in the limited chambers. The flow was unconditionally unstable in a fine-over-coarse stratified sublayer and conditionally unstable in the homogeneous sand under the effects of air entrapment and surface desaturation. In multiple-layered systems, infiltration flow was semiunstable; fingers developed in the sand layer and were stabilized in the loam. In the repellent sand the wetting front was unstable under low ponding conditions; however, it was stabilized when the ponding depth exceeded the water-bubbling (entry) value of the hydrophobic medium. Both the velocity and pressure head criteria predicted fingering in the sand (layers) with the effects of gravity. However, the criteria failed to predict stable flow in the loam, indicating that the capillary (stabilizing) effects on the flow need to be included in theoretical developments. Finally, the observed width and speed of the fingers and the system flux were found to be always higher under air-draining fingering conditions than with fingering under air-confined conditions. ¿ 1998 American Geophysical Union