A flow reactor fluidic module (12) includes a reactant fluid module (20) having an internal process fluid passage (22) and a first major planar outer surface (24a) and a thermal resistance R between the internal process fluid passage (22) and the first major planar surface (24); a thermal control fluid module (30) having an internal thermal control fluid passage (32) and a second major planar outer surface (34a); a holding structure (50) holding the reactant fluid module (20) and the thermal control fluid module (30); and a gap (25) separating the first major planar surface (24a) from the second major planar surface (34a). The gap (25) comprises an interchangeable or replaceable substance or sheet (26) having a thermal resistance G across the gap (25), wherein G is not equal to R.