The results of a study aimed at identifying and understanding possible causes of seasonal variations in NOx, emissions from circulating fluidized-bed (CFB) coal-fired power plants are. discussed. Historical data on fuel properties, operating conditions, and emission characteristics provided by two plants were used. The extensive data were carefully and thoroughly examined and analyzed for any evidence of seasonal variations in NOx and any of the variables and relationships between NOx and the pertinent parameters. Prior to the comprehensive data analyses, a review of the literature was conducted to identify the NOx dependence parameters or variables of importance. Results obtained reveal that relatively higher ammonia was required to control NOx in the winter compared to the rest of the year at the plant that uses this compound to control NOx. NOx emission and a number of operating conditions/factors (bed temperature, excess oxygen, cyclone outlet temperature, and Ca/S ratio) were observed to vary unpredictably with season, whereas a predictable seasonal variation was observed for a few other factors (humidity ratio, ammonia consumption, fuel moisture content, and ambient air temperature). Bed temperature, excess oxygen, cyclone outlet temperature, Ca/S ratio, and ammonia consumption were suspected to be related to the variations in NOx emission.