  Transient microstructure in the diffuse interstellar medium has been observed
towards galactic and extragalactic sources for decades, usually in lines of
atoms and ions, and, more recently, in molecular lines. Evidently, there is a
molecular component to the transient microstructure. In this paper we explore
the chemistry that may arise in such microstructure. We use a PDR code to model
the conditions of relatively high density, low temperature, very low visual
extinction and very short elapsed time that are appropriate for these objects.
We find that there is a well defined region of parameter space where detectable
abundances of molecular species might be found. The best matching models are
those where the interstellar microstructure is young (<100 yr), small (~100
AU), and dense (>10^4 cm^-3).
