An innovative type of engineered composite polymer sheet with enhanced thermal conductivity is described and tested. The concept is to sandwich a closed-loop serpentine channel, cut out in a polypropylene sheet and containing a self-propelled liquid-vapour mixture, between two sheets of the same material bonded by selective laser welding. Polymer materials are widely used to replace metals in different applications, however sometimes this is not possible because of their poor thermal conductivity. Thin, flexible and low-weight polymer sheets can be advantageous in various aerospace, aircraft and portable electronic applications where the device weight and its mechanical flexibility are crucial. The transient and steady-state thermal response of a prototype engineered polymer sheet was characterised for different heat power levels and spatial orientations. The equivalent thermal conductance, calculated from the surface temperatures at opposite ends of the sheet, increases four to six times in comparison with a composite polypropylene sheet without working fluid.