This study investigated the effects of processing three types of texts, namely expository texts, narrative texts, and a combination of both known as twin texts, on incidental vocabulary acquisition and retention in L2 learners. College freshmen (N = 109), who were lower to upper intermediate learners of English, were assigned into a control group and three experimental groups representing different reading conditions. Over a period of seven weeks, the experimental groups engaged in two reading sessions to read their assigned texts accompanied by output tasks and input enhancement. The participants' receptive and productive vocabulary gains were measured with an immediate posttest and a delayed posttest to assess retention. Results revealed significant impacts of text genres on L2 incidental vocabulary learning and retention. Processing twin texts and expository texts yielded significantly greater vocabulary gains than processing narrative texts alone. These effects might be due to different processing mechanisms required for texts structured in distinct ways. Expository texts are informative and explanatory in nature, providing explicit and contextually rich information for word comprehension and retention.