Signs used for science interpretation within national parks have been little studied. We analyzed the textual content of 129 signs in 11 US national parks. Science content was high, but readability was low overall and inversely related to the amount of science content. The amount of science varied by subject area and national park, as did the depth of information and its relevance to humans. Colorado’s Great Sand Dunes National Park, however, had signs containing high amounts of science with the highest readability scores, emphasizing the potential benefits for science communication that can come from understanding the science of signs.