A new two-parameter RTD model based on the one-parameter laminar flow model has been proposed. The model, 'n-laminar model', is defined in time domain and considers a generalization of the parabolic velocity profile across radial direction; its mathematical deduction is presented in text. The model has been validated for both supercritical and near critical CO2 and near critical H2O. It is shown how the proposed two-parameter model works much better than the classical models with one, two or even three parameters for both CO2 and H2O under near critical and supercritical conditions. A range of experiments at 10-30 MPa and 100-250 degrees C at different flowrates are presented. Traditional models, such as n-tanks in series or a combination of n-tanks with a plug flow resulted in a poor explanation of the behaviour in most cases with average errors over 100%. Laminar flow has shown the best results within all these classical models, with a mean average error of 50%. The proposed model predicts with an average error of less than 10-20%. Thus the generalization of the laminar flow to n-laminar is a significant improvement over traditional models. This model is the first successful attempt for the modelling of RTD curves at high pressures. (c) 2006 Elsevier B.V. All rights reserved.