In this paper, the author discusses the appropriateness of treating explanatory variables of a single-equation error correction model as being weakly exogenous. He restates the derivation of the error correction model, and particularly the single-equation conditional error correction model, both in reduced and structural form. These allow the author to point out that the condition of weak endogeneity implicitly tested by usual orthogonality tests is generally not sufficient for single-equation conditional error correction models. The potential pitfalls of working within incomplete simultaneous dynamic equation models for exogeneity tests are also pointed out. The author illustrates this on some empirical studies. Copyright 1992 by Blackwell Publishing Ltd
