  Recent results obtained from studies of diffractive processes in hard
photoproduction performed by the ZEUS collaboration using data delivered by
HERA in 1993 and 1994 are presented. In particular, we have found that $(7 \pm
3)$\% of events with two jets at a pseudorapidity interval of 3.5 to 4 are
inconsistent with a non-diffractive production mechanism. These events may be
interpreted as arising due to the exchange of a colour singlet object of
negative squared invariant mass ($-t$) around 40~GeV$^2$. We have also probed
the structure of the exchanged colour singlet object in low--$t$ diffractive
scattering. By comparing the results from photoproduction and electroproduction
processes we find that between 30\% and 80\% of the momentum of the exchanged
colour singlet object which is carried by partons is due to hard gluons.
