We have examined the thermal decomposition of formic acid (HCOOH) adsorbed on a Pd(100) surface using thermal desorption spectroscopy (TDS) and Fourier transform reflection-absorption infrared spectroscopy (FT-RAIRS). The first layer of adsorbed HCOOH starts to decompose at 170 K. The main decomposition products are carbon monoxide, carbon dioxide, and hydrogen. Carbon dioxide desorbs upon formation between 170 and 220 K. We have no experimental evidence for a decomposition intermediate containing more than two atoms.