Purpose: The purpose of this study is to demonstrate how to use customer spontaneous comments to identify which aspects influence the overall customer satisfaction with restaurant services from a nonlinear perspective. Design/methodology/approach: The authors collected data from 399 spontaneous comments about a chain of fast-food restaurants in Brazil. The comments are freely available on the TripAdvisor portal and were extracted and classified according to seven dimensions related to the quality of services: tangibles, reliability, responsiveness, assurance, empathy, quality of the food and price. Next, the authors combine the critical incident technique (CIT) and the penalty-reward contrast analysis (PRCA) to investigate the nonlinear relationship between service quality assessment and overall customer satisfaction. Findings: The method of integrating CIT with PRCA explains 64.7% of the variation in the customer's assessment of the services provided (R²_Adj = 0.647). This shows that spontaneous comments from customers are related to their overall satisfaction with the service provided. Besides, the findings suggest that consumers tend to comment more about positive than negative experiences regarding aspects related to food, attendants' empathy and service assurance, and more negative comments about aspects related to responsiveness and price. However, it was found that negative comments have a stronger influence on overall satisfaction than positive comments. Originality/value: Using comments available for free on the Internet and evaluating how positive and negative comments can jointly influence customer satisfaction, the proposed methodology demonstrates how restaurants can use their customers' spontaneous comments to identify critical aspects to be managed and improved. To the best of authors’ knowledge, this is the first study presenting how restaurants can use customer spontaneous comments, freely available on the internet, to identify the relevance of different aspects of the services provided from a nonlinear perspective. In addition, the present study shows that although customers spontaneously tend to share more positive than negative comments about restaurant services, events related to negative experiences have a stronger influence on overall satisfaction. © 2021, Emerald Publishing Limited.