This study deals with the decomposition of ethylene carbonate (EC) by H2O in the absence and presence of catalytically active hydroxide ions (OH-) at reaction conditions close to lithium-ion battery operation. We use On-line Electrochemical Mass Spectrometry (OEMS) to quantify the CO2 evolved by these reactions, referred to as H2O-driven and OH--driven EC hydrolysis. By examining both reactions at various temperatures (10 - 80 degrees C) and water concentrations (<20 ppm or 200, 1000, and 5000 ppm H2O) with or without catalytically active OH-ions in EC with 1.5 M LiClO4, we determine an Arrhenius relationship between the CO2 evolution rate and the cell temperature. While the apparent activation energy for the base electrolyte (<20 ppm H2O) is very large (app. E-a approximate to 153 kJ/mol), substantially lower values are obtained in the presence of H2O (app. E-a approximate to 99 +/- 3 kJ/mol), which are even further decreased in the presence of catalytically active OH-(app. E-a approximate to 43 +/- 5 kJ/mol). Our data show that OH--driven EC hydrolysis is relevant already at room temperature, whereas H2O-driven EC hydrolysis (i.e., without catalytically active OH-) is only relevant at elevated temperature (>= 40 degrees C), as is the case for the base electrolyte. Thus, catalytic quantities of OH-, e.g., from hydroxide contaminants on the surface of transition metal oxide based active materials, would be expected to lead to considerable CO2 gassing in lithium-ion cells. (C) The Author(s) 2016. Published by ECS. All rights reserved.