Ensuring a high level of customer satisfaction with public transport is a key objective for transit agencies wishing to maintain and/or increase service uptake. Importance – Performance Analysis (IPA) can be used to identify gaps in the performance of public transport service attributes relative to their importance. However, this approach is often used without adequate consideration of its validity or reliability. Using a case study of bus services in Tehran, Iran, this paper contributes to the literature by improving the validity and reliability of IPA through: incorporating Exploratory Factor Analysis (EFA) to identify key components of bus service attributes, proposing a method for prioritising attributes, introducing confidence intervals around each attribute to aid in the interpretation of results, and dividing the sample using Cluster Analysis (CA) to mitigate the effects of heterogeneity on the results. The performance and importance of 15 different bus service attributes was assessed through a survey of 390 bus passengers conducted at six high-traffic bus terminals in Tehran. EFA results showed that the bus attributes could be represented by four key components: convenience, comfort, reliability and security. CA results classified the sample into three clusters based on socio-demographic and bus travel characteristics. IPA results indicated that transit agencies and policy-makers need to give greater attention to improving comfort related attributes of bus services in Tehran such as ventilation, cleanliness of buses, and providing facilities for elderly/disabled people. The enhanced approach to IPA developed in this paper can be applied to other jursidictions where agencies are seeking to identify areas to improve public transport customer satisfaction.