Strong and broad emission bands commonly attributed to polycyclic hydrocarbonsdominate the mid-infrared spectrum of interstellar matter. Early photo- andspectrometry by the ISO and IRTS missions has proved that these bands arewidespread in the diffuse interstellar medium. We have sampled the galacticdistribution of unidentified infrared band emission in the diffuse ISMcomponent. The remarkable insensitivity of the shape of the UIR band spectrum isseen in this sample; little variation with line strength or galactic positionis seen suggesting either a homogeneous history of the UIR band carriers withinthe Milky Way, convergent evolution of grains suppressing any differences seen inyounger populations or an emission mechanism highly insensitive to particle type and physical conditions.