Language is critical to the effectiveness of marketing messages. Achieving a desired outcome requires arranging words to formulate a message (i.e., syntax), but this task is not trivial. Here, we study the role of syntax in marketing communications by focusing on syntactic surprise: how unexpected the syntax of a message is. The paper introduces a measure that captures syntactic surprise, establishes its internal and external validity, and tests its effectiveness for marketing messages. In a series of studies that include field data and randomized field experiments from contexts such as donations, advertising, and product reviews, we show that a message’s syntactic surprise is related to its effectiveness. This relationship follows an inverted U-shape, such that medium syntactic surprise messages are the most effective. We then conduct experiments on Facebook and Instagram to demonstrate how these findings can be used to write effective marketing messages. In collaboration with two independent companies, we show that ads for products, services, or jobs that are written with medium syntactic surprise result in higher click-through rates than ads written with low or high syntactic surprise.