This paper examines the membership of the professionalassociation of chemical engineering in Britain – the Institutionof Chemical Engineers – during its first three decades. Usingcollective methods of biography, it explores how long it took forclear boundaries to develop between this membership and the widerchemical community. Delineation was linked to the development ofan academic discipline. This paper argues that the indeterminateconstituency of the IChemE delayed growth, but allowed it to playa key role in shaping the development of the new profession.