Polar molecules in selected quantum states can be guided, decelerated and trapped using electric fields created by microstructured electrodes on a chip. Here we demonstrate that transitions between two vibrational quantum states can be induced while the molecules are trapped above the chip. We use CO molecules, prepared in the J = 1 rotational level of the a a(3)Pi(1), v = 0 state and induce the transition to either the J = 1 or the J = 2 level in the vibrationally excited a(3)Pi(1), v = 1 state with pulsed, narrowband IR radiation. First, the vibrational excitation is studied using CO molecules in a freely propagating molecular beam, in a well-defined homogeneous electric field. Then, we demonstrate that the IR radiation can be coupled to CO molecules that are trapped less than 50 mu m above the chip by guiding molecules in the v = 0 level to the center of the chip where they are pumped to the v = 1 level. The molecules remain trapped and are then guided off the chip and state-selectively detected.