Several fibre–reactive basic (polymethine) dyes containing the N–chloroacetyl group have been prepared. They had better washing fastness on wool than their non–reactive analogues.Exhaustion figures were obtained in the usual way (colorimetrically) for all the dyes in Table 1 at the following pH: 2. 5, 3. 6, 4. 7, 5. 8, 6. 9 and 8. 4. However, these results were not in accord with visual assessments of the depths and it appears likely that some decomposition of the dyes takes place,in the presence of wool, particularly at higher pH v