The paper describes some simple techniques for estimating the sampled impulse response of a time-varying channel, in the case where a maximum-likelihood detector is used at the receiver, and where severe intersymbol interference is, from time to time, introduced into the received signal. The estimators are studied for the particular application of a synchronous serial data-transmission system operating at 2400 bit/s with a 4-level quadrature amplitude-modulated signal transmitted over a model of an HF radio link, the latter having two independent Rayleigh-fading sky waves and frequency spreads of 1/2;, 1 and 2 Hz. Results of computer simulation tests are presented, comparing the accuracies of the estimates of the channel given by the different estimators, and hence suggesting the most suitable estimator.