Four experiments are reported, investigating the mechanisms underlying the compatibility effect in the Implicit Association Test (IAT; Greenwald, McGhee, & Schwartz, 1998). All experiments involved two IATs: flower IAT, in which the target categories were flowers and insects, and number IAT, involving even and odd numbers as target categories. In Experiment 1, using the standard IAT procedure, the two IATs produced equal IAT effects, despite large differences in rated valence contrast between flowers and insects and between even and odd numbers. Experiment 2 used a procedure developed by Klauer and Mierke (2005) and produced results consistent with the view that valence plays a role in the flower IAT but not in the number IAT. Experiments 3 and 4 used manipulations similar to those developed by Rothermund and Wentura (2004) and showed that these manipulations affected the flower IAT and number IAT differently. The results provide converging evidence that the two types of IAT effects-one based on valence and one based on familiarity-are empirically dissociable. Experiments 1 and 2 reported in this paper were conducted as an honours project by Marie Peek-O'Leary under the supervision of Sachiko Kinoshita. Parts of this paper were presented at the 44th Annual Meeting of the Psychonomic Society held in Vancouver, Canada, in November, 2003.