The dimensionless how resistance in the corners of capillary tubes (Ransohoff and Radke, J. Colloid Interface Sci. 121, 392, 1988), was applied to the imbibition of a wetting liquid in the corners of a square capillary tube occupied predominantly by nonwetting air. Experimental results of imbibition of a slug of a wetting fluid introduced in a 300- and a 500-mu m square capillary tube are presented. Five different liquids with various surface tension and viscosity values were used. It is shown, both theoretically and experimentally, that the imbibition rates are in proportion to the (sigma/mu)(1/2). The imbibition rate decreases greatly when the spreading fluid does not wet perfectly the tube wall. This is mainly due to the cross-section area available for the flow of the wetting fluid in the corners becoming smaller as the contact angle increases. It was also found that the imbibition rate of a wetting fluid in a square capillary tube decreases with decreasing tube size. Theoretically, the imbibition velocity is proportional to D-1/2, where D is the tube size.