A micro-reactor system assembly comprises a stack of at least n process modules (1-6), wherein n is an integer equal to or greater than 1, made from a rigid first material and comprising at least one reactive fluid passage (1A, 1B, 2A, 3A, 6A) for accommodating and guiding a reactive fluid, and at least n+1 heat exchange modules (7, 8) made from a ductile second material other than said first material and comprising at least one heat exchange fluid passage (7A, 8A) for accommodating and guiding a heat exchange fluid, wherein each process module (1-6) is sandwiched between two adjacent heat exchange modules (7, 8).