Bubble nucleation is a key step in the polymer foaming process using physical agents. Understanding bubble nucleation is vital in order to predict the foam structure based on process conditions. In this work, the influence of sample properties and operating conditions on the pressure drop threshold, Delta P-threshold, which is the difference between the saturation pressure and the ambient pressure at the onset of bubble nucleation, was studied for polypropylene (PP), low-density polyethylene (LDPE), and polystyrene (PS) using CO2 and N-2 as foaming agents. As a result, the Delta P-threshold was a linear function of the interfacial tension regardless of the foaming gas used for each polymer. Moreover, by correlating the experimental results of bubble number density with the Blander-Katz bubble nucleation rate equation, the pressure inside a critical bubble, P-G,P-cr, was studied. The pressure difference between the inside and outside a critical bubble strongly depended on both the interfacial tension and the temperature. According to these relationships, the bubble nucleation rate could be estimated using interfacial tension for all the polymer/gas systems examined. (C) 2015 Elsevier B.V. All rights reserved.