Temperature changes of micro-optical elements can be expected in applications such as telecommunication and laser machining. We present a comparative study of diffractive gratings fabricated in benzocyclobutene (BCB) and in a conventional photoresist under the influence of elevated temperature. We measured variations in both optical diffraction efficiency and topography by heating the samples to 300 degrees C in air in a specially built oven. Our main findings show that gratings fabricated in BCB exhibit small changes, both optically and topographically, during the first 45 min, whereas the conventional photoresist gratings change drastically within just a few minutes.