A process for producing microfluidic articles comprises (a) preparing a photoreactive composition comprising (1) at least one reactive species that is a polymer or a polymer precursor and that is capable of undergoing an acid- or radical-initiated chemical reaction and (2) at least one multiphoton photoinitiator system; (b) exposing a portion of the composition to light sufficient to cause simultaneous absorption of at least two photons, thereby forming exposed and unexposed portions of the composition, and thereby inducing at least one acid- or radical-initiated chemical reaction in the exposed portion; and (c) removing either the exposed or the unexposed portion of the composition, so as to form a microfluidic article comprising a seamless polymer matrix that defines at least one inlet, at least one outlet, and a microfluid processing architecture that is capable of fluidic communication with the inlet and the outlet and that is otherwise fully enclosed within the polymer matrix.