This paper investigates the determinants of delays on international and domestic routes of Turkish Airlines in 2014, focusing on the effects of airport market share and airport market concentration. When examining the impact of airport market share, we find that delays are decreasing in market share for both domestic and international routes. We use market concentration to test whether airlines internalize the cost of the delays for their passengers. We find results that are consistent with the internalization hypothesis on domestic routes; however, on international routes, our estimates are inconsistent with the internalization hypothesis. We propose that these results may be due to market share being more critical than market concentration in determining delays and that the internalization effect is limited. Additionally, we find that delays are lower at airports that connect to more destinations. (C) 2017 Elsevier Ltd. All rights reserved.