1. Introduction A large body of literature during the past two decades has considered the impact of integrated time series in econometric research (cf. surveys by Diebold and Nerlove, 1990; Campbell and Perron, 1991). In univariate analysis, the Box–Jenkins (Box and Jenkins, 1970) approach of studying di>erence-stationary ARMA models requires a consistent and powerful test for the presence of unit roots. In general, such tests have non-standard limiting distributions; for example, the original Dickey–Fuller test (Dickey and Fuller, 1979) and the subsequent augmented Dickey–Fuller (ADF) test statistic (Dickey and Fuller, 1981) converge to a function of Brownian motion under quite general conditions (Said and Dickey, 1984). The critical values of the empirical distribution were "rst tabulated by Dickey (cf. Fuller, 1976). Semi-parametric test procedures have also been proposed (i.e. Phillips, 1987; Phillips and Perron, 1988), with improved empirical size and power properties under certain conditions (cf. Diebold and Nerlove, 1990).