Abstract This paper investigates how software composition and transformation can be applied to domain specific languages used today in modeling and simulation of physical systems. More specifically, we address the composition and transformation of the Modelica language. The composition targets the ModelicaXML dialect which is the XML representation of the Modelica language. By extending the COMPOST concrete composition layer with a component model for Modelica, we provide composition and transformation of Modelica. The design of our COMPOST extension is presented togheter with examples of composition programs for Modelica.