Photoactiveorganicthin films are attracting rapidly growing attention. Model investigations are aiming at establishing a new era of electronics, based on molecular materials – with the potential of miniaturization down to the molecular level (“molecular electronics”). As a fundamental difficulty, however, electric, photoelectric and optoelectronic properties of organic thin film samples are often found to vary considerably with the preparation parameters. – Using vapor-deposited 3,4,9,10-perylene-tetracaboxylic- dianhydride (PTCDA) thin films and measuring the electron time-of-flignt mobility we prove that, what appears as poor reproducibility, largely is a consequence of a wide scatter of structural parameters, such as the degree of cystallinity and orientational order. We find that the electron mobility – over orders of magnitude – is (anti-)correlated with the width of the X-ray rocking curve of the respective sample, reaching 3×10-2cm2/Vs for our presently best one.