Trans:(CH)x:n‐CdS heterojunctions have been fabricated and used to study the properties of undopedtrans‐polyacetylene. TheI‐Vdata show rectifying behavior characteristic of ap‐njunction diode, thus confirming that as‐grown polyacetylene isptype. FromC‐Vcharacteristics we infer a residual acceptor concentration in as‐grown films ofp‐(CH)xofNA?2×1018cm−3. Depletion has been studied in the polymer by using CdS of different doping concentrations. Detailed studies of the photovoltaic response at energies belowEgfor (CH)ximply the existence of a meta‐stable deep trap in the polymer with energy near the center of the gap. The threshold energy for pumping into this level provides an independent measurement of the energy gap,Eg=1.45 eV.