A wire-mesh sensor, which is based on local conductivity measurement, has been applied to studies on the characteristics of bubble flow in a rectangular channel (20 x 100 mm(2)). Special design of the sensor allowed the measurement of the local instantaneous true gas velocity besides the measurement of the local instantaneous void fraction. A review of an already published method for true gas velocity measurement under consideration of the uncertainty caused by limitations in the sampling frequency is presented. A cluster-algorithm is proposed for the evaluation of bubble size distribution and volume flow reconstruction. The validity of this algorithm for spatial field reconstruction was benchmarked by theoretical considerations as well as comparison of the calculated with alternatively measured data. Good agreement was stated. The achieved information was used to obtain plots showing the bubble/slug velocity (up to the second statistical momentum) depending on the spherical-equivalent bubble diameter. This information was measured inside a transient bubble flow with void fraction of up to 20%. Occurring phenomena are explained by presented Fourier spectra of the cross-sectional averaged void fraction and the gas volume flow.