The problem studied in this paper is one of improving the performance of a class of adaptive observer in the presence of exogenous disturbances. The 1100 gains of both a conventional and the newly proposed sliding-mode adaptive observer are evaluated, to assess the effect of disturbances on the estimation errors. It is shown that if the disturbance is "matched" in the plant equations, then including an additional sliding mode feedback injection term, dependent on the plant output, improves the accuracy of observation. (C) 2016 Published by Elsevier Ltd.