This invention provides composite plastic articles and methods of making them. The articles can be fluidic or microfluidic devices having fluidic conduits and, optionally, pneumatic conduits that regulate flow in the fluidic conduits. The articles comprise a first substrate coated with a layer of a material that comprises, or onto which have been introduced, reactive groups. For example, the substrate can be a plastic coated with an oxide or a siloxane onto which hydroxyl groups have been introduced. These articles are covalently bonded with other articles comprising reactive groups on their surfaces, for example, polysiloxanes treated to have silanol groups. Certain articles have specified locations on their surfaces that are not bonded to the other piece. For example, the coating can be removed from these locations before bonding. Such locations can be useful as functional elements of various devices, such as valve seats in valves of microfluidic devices.