Various Rochow contact masses have been characterized by catalytic tes
ts as well as by X-ray diffraction with respect to eta-Cu3Si percentag
e and particle size. The presence of eta-Cu3Si in the contact masses i
s neither a necessary nor a sufficient precondition of catalytic activ
ity, but a certain contribution of dispersed eta-Cu3Si to the activity
cannot be excluded. The results can be explained consistently by assu
ming highly dispersed or even two-dimensional Cu-Si species, not detec
table by X-rays, as catalytically active species. The promoters zinc,
antimony and tin do not significantly change the percentage and partic
le size of eta-Cu3Si.