Recent studies have revealed that water‐dispersible colloids play an important role in the transport of nutrients and contaminants in soils. In this study, water‐dispersible colloids extracted from saline–alkali soils have been characterized by atomic force microscopy (AFM), X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), and UV absorption spectra. AFM observation indicated that the water‐dispersible colloids contain some large plates and many small spherical particles. XRD, XPS, and UV absorption measurement revealed that the water‐dispersible colloids are composed of kaolinite, illite, calcite, quartz and humic acid. In addition, UV absorption measurement demonstrated that the humic acids are associated with clay minerals. Water‐dispersible colloids in the saline–alkali soils after hydrolyzed polymaleic anhydride treatment and an agricultural soil (nonsaline–alkali soil) were also investigated for comparison. The obtained results implied that the saline–alkali condition facilitates the formation of a large quantity of colloids. The use of AFM combined with spectrometric methods in the present study provides new knowledge on the colloid characteristics of saline–alkali soils. Microsc. Res. Tech. 79:525–531, 2016. © 2016 Wiley Periodicals, Inc.