Purpose: To investigate factors that may affect the release of 5-ASA from MMX™ mesalamine (Lialda®; Shire Pharmaceuticals Inc., Wayne, PA, USA) and other pH-dependent delayed-release 5-ASA formulations for the treatment of ulcerative colitis (UC). Published literature was evaluated and considered in light of data from in vitro and in vivo studies. Methods: Study 1 (scintigraphic): eight healthy subjects were randomized to MMX mesalamine or a pH-dependent, delayed-release mesalamine formulation (Asacol®[Giuliani SpA, Italy]) in an open-label, two-way cross-over study. Study 2 (dissolution): three batches of MMX mesalamine tablets were analyzed using non-destructive three-dimensional terahertz pulse imaging to determine film coat thickness and morphology. Dissolution testing using USP II apparatus (50 rpm) was subsequently performed at pH 6.8 and 7.2 (found in the GI tract of UC patients) using the technique reported by Spencer et al (J Pharm Sci, 2007) for Asacol® delayed-release tablets (P&G, Cincinnati OH, USA). Results: Study 1: initial disintegration of both tablets generally occurred between the distal small bowel and the ascending colon. Initial tablet disintegration (mean hours ± standard deviation [SD]) occurred earlier in the GI tract for MMX mesalamine (4.75 ± 1.31) than for the comparator formulation (6.16 ± 1.80). Complete disintegration occurred later in the GI tract for MMX mesalamine compared with the comparator formulation (17.37 ± 8.63 vs 7.27 ± 2.13). Study 2: the mean coating thickness (μm ± SD) of MMX mesalamine tablets ranged from 109.2 (±16.8) in batch 1 to 113.8 (±19.8) in batch 3. MMX mesalamine tablets demonstrated steady release of 5-ASA at both pH values, with complete release achieved over > 8 hours at pH 6.8 and over > 12 hours at pH 7.2. Spencer et al. (2007) reported Asacol coating thicknesses ranging from 76.8 (±18.2) in batch 1 to 80.5 (±12.5) in batch 3. 5-ASA release was consistent over the initial 2 hours at pH 7.2, but was highly variable at pH 6.8. Between 80–100% tablet disintegration occurred within 2 hours. Conclusion: MMX mesalamine demonstrated steady 5-ASA release at pH 6.8 and 7.2 in vitro and in vivo using scintigraphic analysis. This was not reported for Asacol (Spencer et al. 2007). Together, these data suggest a consistent, steady release of 5-ASA from MMX mesalamine in the colon, while the comparator formulation appears to release the majority of its 5-ASA load in the ascending colon.