Although, poly(dimethylsiloxane) (PDMS) is a widely used material in numerous applications, such as micro- or nanofabrication, the method of its selective etching has not been known up to now. In this work authors present two methods of etching the pure, additive-free and cured PDMS as a positive resist material. To achieve the chemical modification of the polymer necessary for selective etching, energetic ions were used. We created 7 mu m and 45 mu m thick PDMS layers and patterned them by a focused proton microbeam with various, relatively large fluences. In this paper authors demonstrate that 30 wt% Potassium Hydroxide (KOH) or 30 wt% sodium hydroxide (NaOH) at 70 degrees C temperature etch proton irradiated PDMS selectively, and remove the chemically sufficiently modified areas. In case of KOH development, the maximum etching rate was approximately 3.5 mu m/min and it occurs at about 7.5 x 10(15) ion x cm(-2). In case of NaOH etching the maximum etching rate is slightly lower, 1.75 mu m/min and can be found at the slightly higher fluence of 8.75 x 10(15) ion x cm(-2). These results are of high importance since up to this time it has not been known how to develop the additive-free, cross-linked poly(dimethylsiloxane) in lithography as a positive tone resist material.