This Letter reports an experimental study which achieves reproducible oscillations in both pH and heat output (Qr) during a palladium-catalysed phenylacetylene oxidative carbonylation reaction in an homogeneous catalytic system (PdI2-KI-Air-NaOAc in methanol solution). Experiments were performed in an HEL SIMULAR (TM) reaction calorimeter with precise control of temperature and gas flow rates. Under certain experimental conditions a second set of oscillations was observed. Oscillations in Qr were exothermic with no corresponding endotherm. Total energy released during oscillations followed a staircase function with a maximum of 600 J/oscillation. Heat release was in phase with pH fall, and decreased as pH increased. (c) 2006 Elsevier B.V. All rights reserved.