Spirobi(dithienosilole)s were prepared and their optical and FET properties were studied. They showed absorption maxima at 358-368 nm, a little red shifted from those of the corresponding non-spiro type dithienosiloles, indicating that spiro-conjugation operates in these molecules to an extent. The field-effect hole mobility of spirobi[bis(trimethylsilyl)dithienosilole] was determined to be 1.4 x 10(-6) cm(2) V-1 s(-1).