Flow characteristics of a countercurrent gas-liquid packed column were investigated using radioactive labeling of both phases. External scintillation detectors were used to record the tracer concentration without disturbing the flow pattern. Detectors located at a given bed height showed that flow was nearly axisymmetrical. Detectors at different bed heights gave local residence time distributions (RTD) which were interpreted by the one-dimensional dispersive plug flow model. The liquid velocity was found to be nearly uniform over the column. Conversely, the gas velocity and the dispersion coefficients of both phases show significant length dependence. This could explain the heterogeneity of the existing correlations which are mainly based on global (i.e. input/output) RTD analysis. The non-linear increase of the RTD variance with bed height also shed a doubt on the validity of the dispersion concept. The proposed method thus revealed to be a powerful tool for non-intrusive investigation of a complex flow pattern in a full-scale column.