Ambient PM2.5 was sampled in three indoor environments (a common office, a photocopy room and a student dormitory) and one outdoor environment (a rooftop) on a campus of Nanchang University in Nanchang city, China, on June 5–20, 2009. Analysis by ion chromatograph showed the indoor-outdoor differences and relationships of water-soluble ions. The indoor-outdoor distributions indicated that chemicals and household garbage contributed most to the generation of water-soluble ions indoors. The indoor-outdoor relationships of water-soluble ions in PM2.5 in the common office and student dormitory were unrelated or weak, which indicated that the ions were produced indoors rather than carried in from outdoors. Correlations between various water-soluble ions in indoor and outdoor PM2.5 are discussed here, and it is suggested that the photocopier machine contributed little to the water-soluble ions indoors. Ion balance calculations indicated that the anions and cations in the photocopy room and outdoor environment shared an origin, but part of the anions and cations in the common office room and student dormitory originated indoors. The linear regression slopes (anion/cation) are all lower than 1, with the anion deficits probably affected by the absence of data on F−, PO43−, NO2−, CO32− and organic acid salt.