An integrated assembly includes an insulative mass with a first region adjacent to a second region. The first region has a greater amount of one or more inert interstitial elements incorporated therein than does the second region. Some embodiments include an integrated assembly which has vertically-extending channel material pillars, and which has memory cells along the channel material pillars. A conductive structure is under the channel material pillars. The conductive structure includes doped semiconductor material in direct contact with bottom regions of the channel material pillars. An insulative mass is along the bottom regions of the channel material pillars. The insulative mass has an upper region over a lower region. The lower region has a greater amount of one or more inert interstitial elements incorporated therein than does the upper region. Some embodiments include methods of forming integrated assemblies.