Signed languages exploit the visual/gestural modality to create iconic expression across a wide range of basic conceptual structures in which the phonetic resources of the language are built up into an analogue of a mental image. Previously, we demonstrated a processing advantage when iconic properties of signs were made salient in a corresponding picture in a picture/sign matching task (Thompson et al., 2009). The current study investigates the extent of iconicity effects with a phonological decision task (does the sign have straight or bent fingers) in which the meaning of the sign is irrelevant. The results show that iconicity is a significant predictor of response latencies with more iconic signs leading to slower responses. We conclude that meaning is activated automatically for highly iconic properties of a sign, and this leads to interference in making form-based decisions. This is supported by the even greater inhibition observed when iconicity specific to a signs handshape was analyzed (phonological decisions involved sign handshape). Thus the current study extends previous work by demonstrating that iconicity effects permeate the entire language system, arising automatically even when access to meaning is unnecessary.