Nonlinear dynamics of non-isothermal viscoelastic flow in an axisymmetric sudden contraction geometry are studied for a highly elastic polyisobutylene (PIB) based polymer solution (referred to as PIB-Boger fluid). Temperature gradient in the flow is generated by the external heating or cooling imposed at the boundaries of the system. The critical conditions for the onset of the elastic instabilities at different wall temperatures are determined by accurate instantaneous measurements of pressure drop across the contraction plane and temperature measurements in the flow. These results along with flow images by streakline photography are presented for various values of wall-to-fluid temperature differences and Deborah numbers (dimensionless flow rates). It is shown chat the thermal effects have profound consequences for the onset, development, and structure of the vortical flows in viscoelastic entry flows. Thermal instabilities are not detected in these flows for the range of flow rates and wall temperatures used in the present observations. (C) 2000 Elsevier Science B.V. All rights reserved.