Published online by Cambridge University Press: 20 November 2018
 Let  
 $X$  be a diffusion process, which is assumed to be associated with a (non-symmetric) strongly local Dirichlet form  
 $(\varepsilon ,\mathcal{D}(\varepsilon ))$  on  
 ${{L}^{2}}(E;m)$ . For  
 $u\,\in \,\mathcal{D}{{(\varepsilon )}_{e}}$ , the extended Dirichlet space, we investigate some properties of the Girsanov transformed process  
 $Y$  of  
 $X$ . First, let  
 $\hat{X}$  be the dual process of  
 $X$  and  
 $\hat{Y}$  the Girsanov transformed process of  
 $\hat{X} $ . We give a necessary and sufficient condition for  
 $(Y,\hat{Y})$  to be in duality with respect to the measure  
 ${{e}^{2u}}m$ . We also construct a counterexample, which shows that this condition may not be satisfied and hence  
 $(Y,\hat{Y})$  may not be dual processes. Then we present a sufficient condition under which  
 $Y$  is associated with a semi-Dirichlet form. Moreover, we give an explicit representation of the semi-Dirichlet form.