An impact of morphology on reflectance of porous silicon was investigated. Depending on the metal-assisted chemical etching conditions the macro- micro structures could be formed. The reflectance properties of various porous silicon structures after ammonia adsorption were investigated. It was shown that increasing of ammonia concentration in the measurement camber leads to an increase of the reflectance. The most sensitive structures for ammonia detection are porous silicon having approximately size of pores - 10-15 μm. A fast response of porous silicon on the adsorption of ammonia molecules may be used for development of new sensors.