This study presents the results of a cross-sectional reading and spelling assessment conducted among 118 Spanish children in 3rd, 4th and 5th grade. The first aim was to explore whether children's use of orthographic knowledge was modulated by lexical variables--word frequency and orthographic neighborhood--or sublexical variables--context-dependent, inconsistent or neutral letters--as well as the developmental pathway of such knowledge in both tasks. The second aim was to provide insight into the type of errors committed by children in order to detect the words and structures that convey most difficulties. Data showed that children rely on sublexical processes more than on lexical ones in reading and writing. Persistent errors in context-dependent and inconsistent letters were evident even in 5th grade, and writing involved greater difficulty in all grades. The presence of other type of errors such as substitutions, omissions or lexicalizations was negligible. Finally, an item analysis revealed that errors were located in low-frequency syllables, particularly in the first position. Data point to specific and persistent difficulties in context-dependent and inconsistent letters that may hinder the consolidation of accurate orthographic word representations in Spanish.