The secondary electron yield (SEY) of two different non-evaporable getter (NEG) samples has been measured 'as received' and after thermal treatment. The investigated NEGs are TiZr and TiZrV thin film coatings of 1 mum thickness, which are sputter deposited onto copper substrates. The maximum SEY delta (max) of the air exposed TiZr and TiZrV coating decreases from above 2.0 to below 1.1 during a 2h heat treatment at 250 and 200 degreesC, respectively. Saturating an activated TiZrV surface under vacuum with the gases typically present in ultra-high vacuum systems increases delta (max) by about 0.1. Changes in elemental surface composition during the applied heat treatments: were monitored by Auger electron spectroscopy (AES). After activation carbon, oxygen and chlorine were detected on the NEG surfaces. The potential of AES for detecting the surface modifications which cause the reduction of SE emission during the applied heat treatments is critically discussed.