The basic questions that guide this study are: (a) what percentage of vocabulary from a passage would a Spanish learner need to know to demonstrate "adequate" (a score of 70 out of 100) comprehension of it? And, (b) what type of curve would best describe the relationship between vocabulary knowledge and reading comprehension? Fifty-three students enrolled in two courses of Spanish as a Heritage Language (SHL) at a metropolitan university read a newspaper article, underlined the unknown vocabulary and then answered a reading comprehension test. Our findings suggest, as in previous studies for English as a Second Language (ESL), that a 98% of vocabulary coverage is needed to show adequate comprehension of an authentic passage. The curve that best describes this relationship was not linear as they concluded but was similar to a logarithmic function, which appears to suggest a relationship that obeys to a law of diminishing returns for Spanish as a Second Language (SSL) reading.