GCN Circular 21209
K. Wiersema, N. Tanvir (Leicester), A. Levan (Warwick) report: We observed the afterglow of GRB 170531B (Evans et al. GCN 21171; Lipunov et al. GCN 21173; Pozanenko et al. GCN 21174) with the 4.2m William Herschel Telescope on La Palma, using the ACAM instrument. We obtained a spectrum, consisting of 4x900 seconds exposure time, starting at 02:55 UT on 1 June 2017. Our wavelength coverage is ~4500 - 9500 A. We detect a small number metal lines (at low S/N) at a redshift consistent with that obtained by de Ugarte-Postigo et al. (GCN 21177). We warmly thank the staff of ING, in particular Raine Karjalainen, for their assistance.