Metal-organic frameworks (MOFs) have been extensively exploreda sa dvanced chemical sensors in recent years. However,t here are few studies on MOFs as acidic gas sensors, especiallyp rotonc onductive MOFs.I nt his work, two new proton-conducting 3D MOFs,{ [Co 3 (p-CPhHIDC) 2 (4,4'-bipy)(H 2 O)]·2H 2 O} n (1)( p-CPhH 4 IDC = 2-(4-carboxylphenyl)-1 H-imidazole-4,5-dicarboxylic acid;4 ,4'-bipy = 4,4'-bipyridine) and {[Co 3 (p-CPhHIDC) 2 (bpe)(H 2 O)]·3H 2 O} n (2) (bpe = trans-1,2-bis(4-pyridyl)ethylene) have been solvothermally prepared and investigated their formic acid sensing properties. Both MOFs 1 and 2 show temperature-and humidity-dependent protonc onductive properties and exhibit optimizedp roton conductivities of 1.04 10 À3 and 7.02 10 À4 Scma t9 8% relative humidity (RH) and 100 8C, respectively.T he large number of uncoordinated carboxylic acid sites, free and coordination water molecules, andh ydrogen-bondingn etworks inside the frameworks are favorable to the proton transfer.B ym easuringt he impedance values after exposure to formic acid vapor at 98 %o r6 8% RH and 25 8C, both MOFs indicater eproducibly high sensitivity to the analyte. The detection limit of formic acid vapor is as low as 35 ppm for 1 and 70 ppm for 2.M eanwhile, both MOFs also show commendable selectivity towards formic acid among interfering solutions. The proton conducting and formic acid sensingm echanismsh ave been suggested according to the structural analysis, E a calculations, N 2 and water vapor absorptions, PXRD and SEM measurements. This work will open an ew avenue for proton-conductive MOFbased impedance sensors and promote the potentiala pplication of these MOFs fori ndirectly monitoring the concentrationsofformic acid vapors.[a] R.