Oligothiophene thin film transistors have recently been shown to respond to organic vapors, suggesting possible applicability in the field of olfactory sensor arrays. Here, we present a study of the correlation between the morphological structure of the active semiconductor thin film and the response to the vapor. The study was carried out by combining the measurement of the transient source-drain current of the transistor under vapor flow with the morphological characterization of the organic thin films by transmission electron microscopy.