This study investigated the influence of working memory capacity on the dynamics of text composition from source, notably the strategy used by writers to consult the sources and the text produced so far. Twenty-five graduate students composed a procedural text explaining how to assemble a model turbine. They were free to consult a documentary source, featuring captioned pictures of turbine parts and assembly steps, at any time. Graphomotor and eye movements were recorded using `Eye and Pen' software with an eye-tracker and digitizing tablet. Results showed that writers with high working memory capacity used a different strategy to explore the visual source, making longer pauses and producing more detailed procedures. The role of working memory in text composition from source is discussed with regard to the number of units that have to be held in memory in order to achieve communicative goals.