This paper studies how changes in the two key parameters of unemployment insurance-the benefit replacement rate (RR) and the potential benefit duration (PBD)-affect the duration of unemployment. To identify such an effect we exploit a policy change introduced in 1989 by the Austrian government, which affected various unemployed workers differently: a first group experienced an increase in RR; a second group experienced an extension of PBD; a third group experienced both a higher RR and a longer PBD; and a fourth group experienced no change in the policy parameters. We find that unemployed workers react to the disincentives by an increase in unemployment duration, and our empirical results are consistent with the predictions of job search theory. We use our parameter estimates to split up the total costs to unemployment insurance funds into costs due to changes in the unemployment insurance system with unchanged behaviour and costs due to behavioural responses of unemployed workers. Our results indicate that costs due to behavioural responses are substantial. Copyright 2006 The Review of Economic Studies Limited.