A technique is described for performing solid phase extraction (SPE) at a semi-microscale. 10 mg of electrospun polystyrene fibers (average diameter 2.7 µm) were packed into a SPE barrel to form a disk (5 mm × 1 mm). The device was employed to evaluate the extraction of four steroids: prednisone, hydrocortisone, cortisone acetate and 19-nortestosterone from water and plasma matrices. The analytes were desorbed from the fibers with 100 µl methanol and monitored by high performance liquid chromatography with a diode array detector (HPLC-DAD). The semi-micro SPE method provided extraction recoveries of 51.14–80.13% in plasma and 66.07–93.43% in water. The breakthrough volumes at 500 ng ml⁻¹ ranged from 200–400 µl for all analytes. At optimal conditions, the four analytes followed an excellent linear relationship in the range 12.5–400 ng ml⁻¹ with coefficients of determination (r²) greater than 0.99 and the limits of detection ranged from 0.75 to 1.29 ng ml⁻¹. Due to its simplicity, it is anticipated that the method will greatly simplify disk solid phase extraction.