Background: Participant information sheets and consent forms (PICFs) used in interventional studies are often criticised for being hard to read and understand. We assessed the readability and its correlates of a broad range of Australian PICFs. Methods: We analysed the participant information sheet portion of 248 PICFs. Readability scores were measured using three formulae: the Flesch Reading Ease, the Flesch-Kincaid Grade Level, and the Simple Measure of Gobbledygook (SMOG). We investigated how various features (including sponsor type and PICF type) correlated with PICF length and readability and examined compliance with other measures known to improve readability. Results: For a sample of 248 PICFs, the mean (standard deviation) Flesch Reading Ease score was 49.3 (5.7) and for the Flesch-Kincaid Grade Level 11.4 (1.1). The mean (SD) SMOG score was 13.2 (0.9). The median document length was 3848 words (8 pages). Commercial PICFs were more than twice as long as non-commercial, but statistically more readable (p = 0.03) when analysed using the SMOG formula. Subgroup analyses indicated that PICFs for self-consenters were statistically more readable than those for proxy consenters. The use of tables, but not the use of illustrations was associated with better readability scores. Conclusions: The PICFs in our sample are long and complex, and only 3 of the 248 achieved the recommended readability score of grade 8 or below. The broader use of best practice principles for writing health information for consumers and the development of more context-sensitive templates could improve their utility.