Metal-organic frameworks (MOFs) have been extensively explored as advanced chemical sensors in recent years. However, there are few studies on MOFs as acidic gas sensors, especially proton conductive MOFs. In this work, two new proton-conducting 3D MOFs, {[Co3(p-CPhHIDC)2(4,4'-bipy)(H2O)]·2H2O}n (1) (p-CPhH4IDC = 2-(4-carboxylphenyl)-1H-imidazole-4,5-dicarboxylic acid;4,4'-bipy = 4,4'-bipyridine) and {[Co3(p-CPhHIDC)2(bpe)(H2O)]·3H2O}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 proton conductive properties. The large number of uncoordinated carboxylic acid sites, free and coordination water molecules and H-bonding networks inside the frameworks are favorable to the proton transfer. By measuring the impedance values after exposed to formic acid vapor at 98% or 68% RHs and 25°C, both MOFs indicate reproducibly high sensitivity to the analyte. The detection limit of formic acid vapor can be as low as 35 ppm for 1 and 70 ppm for 2. Meanwhile, both MOFs also show commendable selectivity towards formic acid among interfering solutions. This work will open a new avenue for proton conductive MOF-based impedance sensors and promote the potential application of these MOFs in indirect monitoring the concentrations of formic acid vapors.