Energy storage devices, such as batteries and flywheels, are promising options for providing operating reserves due to their fast response and low emissions during operation. However, because of the complex nature of power systems, adding energy storage-based reserves to the power system may not necessarily benefit the environment. In this paper, we analyze these impacts in a test system and identify important drivers that affect the environmental outcomes. Dispatch results are obtained by solving an optimal power flow (OPF) problem and are used to determine emissions. We find that the impacts of adding energy storage are highly case-dependent. In systems with high renewable penetration levels and significant renewable curtailment, adding energy storage reduces emissions; in other systems, the impacts on emissions could be positive, neutral, or negative. The analyses presented in this paper show that policies to procure energy storage as a means to reduce emissions may actually lead to increased system-wide emissions if current dispatch algorithms are used. We also explore the impacts of modifying the dispatch algorithm to ensure system emissions with energy storage are no worse than system emissions without energy storage. (C) 2016 Elsevier Ltd. All rights reserved.