Void fraction distributions in horizontal pipes with sudden area contraction have been experimentally investigated. The experiments are aimed at analysing the effect of the singularity characteristics on void fraction profiles and phase distribution. The technique employed to obtain the cross-sectional average void fraction along the test pipes is based on the instantaneous measurement of the electrical impedance of the air-water mixture. The instrumentation set up consists of ring electrode pairs placed on the internal wall of the cylindrical test duct, flush to the pipe surface. Two different test sections, a 70/60 mm i pipe and a 70/36 mm i pipe have been employed. Plug and slug flow regimes were investigated in order to obtain the mean void fraction along the channel in five different locations upstream and downstream the singularity. The analysis of the results, in terms of mean values and probability distributions, shows that the characteristics of the flow restriction deeply modify the how structure upstream and downstream the discontinuity.