Adults' simple addition performance (e.g., 3 + 4 = ?) is faster, more accurate, and more often based on direct memory retrieval (rather than a procedural method, such as counting) when problems are presented in digit format (3 + 4) than written-word format (three + four). A possible explanation is that the mathematical symbol + is more compatible to memory retrieval with Arabic numerals than word numerals. To investigate this, two groups of 42 participants received eight blocks of 72 simple addition problems. For one group, operand format (digits or words) switched across trials within each block and operator (the symbol + or the word plus) alternated between blocks. For the other group, operator switched across trials, whereas operand format alternated between blocks. In the switch-format condition, compatible formats (e.g., 3 + 4, three plus four) were solved by direct memory retrieval more often than were incompatible formats (3 plus 4, three + four). There was no compatibility effect on use of direct memory retrieval when operand format was fixed within blocks and operator format switched across trials. There was also a reaction time (RT) advantage only for digit operands with + relative to plus when format switched, but + facilitated only word problems when operand format was blocked. The results indicate that operand-operator compatibility and format switching had previously unsuspected effects that qualify previous research examining format effects in arithmetic.