This paper draws from cognitive psychology and cognitive neuroscience to develop a preliminary similarity-choice theory of how people allocate attention among information patches on webpages while completing search tasks in complex informational websites. Study 1 applied stepwise multiple regression to a large dataset and showed that success rate for web navigation tasks approaches 100% if a single information patch is highly similar in meaning to the user goal, and success rate falls dramatically if two or more information patches compete for the user's attention and if only one contains a link that leads to accomplishing the search goal. Study 2 (n=82) manipulated the independent variables task difficulty and website design and found statistically significant differences in success rate for both main effects and the interaction. Study 2 also found that the distribution of attention among available information patches was strongly determined by the rank ordering of semantic similarity between user goal and information patch but was not influenced by website designs with very different visual layouts. While these results offer verification of a similarity-choice theory of attention to information patches, caution is warranted in generalising too broadly from these results. (Contains 6 figures and 1 table.)