Unwanted noise is the most frequent cause of complaint by occupiers of dwellings with party walls. Therefore the concept of a cavity party wall (separating wall) for domestic housing, built with loadbearing masonry but without wall ties to connect the two leaves, has generated wide interest. Such a wall would give better sound resistance than equivalent tied structures because ties provide sound pathways between leaves. Strictly, though, an untied wall structure would not comply with the structural stability requirements of the current Building Regulations.This paper describes a series of element tests, followed by a full scale model test, that show that an untied cavity party wall can be a stable structure and achieve satisfactory sound attenuation.This paper will interest manufacturers and suppliers of AAC blocks, and building designers and specifiers. 8 pages.