In this paper an experimental investigation of the thermal performances of gas-to-gas micro heat exchangers operating under different flow configurations (co-current, counter-current, cross flow) is presented and the results are compared with the predictions of the conventional correlations developed for the design of the conventional-sized heat exchangers. A double-layered microchannel heat exchanger has been specifically designed in order to be able to reproduce co-current, counter-current and cross flow gas arrangements. The core is housed in a shell made of polymer; on the contrary, the foil between the hot and cold flow is exchangeable; several foils made with different materials (copper, peek, stainless steel and aluminum) and with different thickness have been investigated in order to put in evidence the effect due to the axial wall-fluid conjugate heat transfer on the thermal efficiency of the micro heat exchanger. The results show that the conjugate heat transfer can be very strong in micro heat exchangers and it tends to reduce the effectiveness of counter-current micro heat exchangers and to increase the effectiveness of cross flow micro heat exchangers. In the case of a co-current arrangement with balanced flows the role of the wall axial conduction can be considered negligible. (C) 2013 Elsevier Ltd. All rights reserved.