Several studies, conducted within a psycholinguistic and psychogenetic theoretical framework, have made it known some of the characteristics related to segmentation between words that children write when in their literacy first stages. This work presents, both, a quantitative and a qualitative analysis of 90 texts written by Mexican children who are in second and fourth grade in elementary school. The purpose of this study is to provide some ideas in order to identify the existence of a possible evolutionary pattern in learning segmentation between words in Spanish writing. This work focuses on the analysis of unconventional word segmentation (SNC) that children make and on the identification of possible criteria that guide these segmentations. The findings confirm the initial hypothesis: unconventional word segmentation observed in children's writing is the result of a rational exercise carried out by children in their effort to understand the nature of the graphic units instead of ` spelling errors'. In evolutionary terms, the results show that children explore, in initial levels, different criteria to separate words, and, as their writing knowledge increases, some of these criteria will be discarded.