In this paper we present an empirical study among 40 participants which investigates the relationship between various factors of user interface aesthetics on the one hand, and the influence of the user interface attributes, symmetry, colorfulness as well as visual complexity on user interface aesthetics on the other hand. The user interface aesthetics will be classified in intuitive aesthetics (1st impression with a presentation time of 500 ms) and reflective aesthetics (reflective long-term impression after a longer presentation). Reflective aesthetics is further classified in classical aesthetics (common attractiveness) as well as expressive aesthetics (creativity). For this study we have set up a corpus of 30 websites which are used as stimulus material. In a multi-step lab experiment, participants rate aesthetics and their subjective impression concerning user interface attributes using questionnaires. We are able to show that the intuitive aesthetic judgment correlates strongly with the reflective judgment. The symmetry of a website positively correlates with all definitions of aesthetics, especially with the classical or traditional interpretation in the sense of attractiveness. Visual complexity can be seen as the strongest predictor for the aesthetic judgement of users and it negatively correlates with all definitions. Concerning colorfulness, a preference for websites of a medium degree of colorfulness for the intuitive as well as the classical aesthetics can be stated. Concerning expressive aesthetics, websites of moderate to high colorfulness receive the best judgments. The relationships which we have found are finally discussed in the context of previous research and some implications for future user interface design are given.