In this work we present a novel two-color differential absorption diagnostic near 5.6 mu m using an external-cavity quantum-cascade-laser (EC-QCL) for formaldehyde (CH2O) detection in high-temperature reacting gases. This diagnostic utilizes the narrow mid-infrared absorption features of CH2O in the R branch of the C=O stretching band. Specifically, the online and offline colors were chosen as 1787.05 cm(-1) and 1787.85 cm(-1), respectively. Compared to the previous Stanford work that targeted the 3.5 mu m band of CH2O, this new diagnostic achieved higher sensitivity, better species selectivity and immunity to interference absorption from other combustion gases, especially small hydrocarbons (e.g. methane). Absorption cross sections of CH2O at the two colors were measured in shock-heated 1,3,5-trioxane/argon mixtures over a wide range of temperatures and pressures: 870 K <= T <= 1800 K, 0.7 atm <= P <= 4.5 atm. These cross section measurements were further expressed as analytical functions of temperature and pressure with 2a statistical uncertainties of +/- 4.9%. Initial application of the diagnostic was demonstrated in a shock tube study of the thermal decomposition of methanol/argon mixture at 1427 K and 2.77 atm, where the measured CH2O time-history was compared with simulations from multiple combustion mechanisms. We expect this new diagnostic to be useful in future combustion studies. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.