Abstract <p>The morphology of permafrost-affected tundra soils developing from loamy substrates, their microstructure, and their main chemical properties during summer and winter seasons are considered. The research objects are represented by the profiles of initial (zero-moment) soils, 7- and 23-year-old soils, and fully developed Early, Middle, and Late Holocene soils. The leading role of cryogenic processes in the formation of the main properties and structural organizations of the soils at different stages of their development is shown. The influence of cryogenic processes is mainly realized through the phenomena of thermal shrinking and ice segregation. The structural organization of the soil profiles differs in summer and winter seasons. The instability of morphological features shaped by cryogenic processes is shown: these features may alter or disappear under the impact of variations in the hydrothermal regime, freezing–thawing processes, and forms of ice segregation. The stability of soil structure is determined by the formation of chemically bound aggregates. The structural and microstructural features of the soils in summer and winter periods also depend on the stage of soil development. The characteristic morphological features of soil diagnostic horizons are formed prior to the development of significant differences in their chemical properties. The cryogenic impact on soil the initial stages of soil formation is mainly realized through microturbation of the uppermost horizons. Later on, the horizontal and vertical migration of organic material may affect the structure and properties of the middle-profile and lower soil horizons. The leading processes of the formation of permafrost-affected soils include physical (cryogenic) disintegration of plant debris and enrichment of the upper parts of the profiles with plant detritus and of the middle and lowermost soil horizons with raw humus owing to cryoturbation.</p>