Flow patterns, counter-current flow limitation (flooding), and gas hold-up (void fraction) in counter-current flow in vertical and inclined channels were experimentally investigated. Tests were performed in a 2 m-long channel with 1.9 cm inner diameter, using air, and demineralized water, mineral and paraffinic oils, covering a surface tension range of 0.0128-0.072 N/m, and a liquid viscosity range of 1 x 10(-3)-1.85 x 10(-1) Ns/m(2). The liquid and gas superficial velocity ranges for the tests with demineralized water were 0 less than or equal to U-1S less than or equal to 54 cm/s and 1 less than or equal to U-GS less than or equal to 299 cm/s, for tests with mineral oil were 0 less than or equal to U-1S less than or equal to 23 cm/s and 1 less than or equal to U-GS less than or equal to 248 cm/s, and for paraffinic oil were 0.15 less than or equal to U-1S less than or equal to 11.6 cm/s and 1 less than or equal to U-GS less than or equal to 224 cm/s, respectively. The examined channel angles of inclination with respect to the vertical line were 0, 30 and 68 degrees. Flooding data were significantly different from pure water results only at very high liquid viscosities. The effect of liquid viscosity on gas hold-up and flow patterns was significant, furthermore, and several existing models and correlations were unable to correctly predict the data trends. With increasing the liquid viscosity the parameter range of the slug Bow pattern expanded for all angles of inclination, and froth how replaced the churn flow pattern in the vertical configuration and it replaced the churn/stratified and semi-stratified patterns in inclined configurations. The churn-stratified flow pattern is predominantly wavy stratified and is interrupted by upward-moving flooding-type waves. Semi-stratified is a periodic pattern where in each period the how regime is initially wavy stratified while liquid accumulates in the bottom portion of the test section and forms a large liquid slug which subsequently moves upwards in the channel.