Although Spanish and Italian voiced stops are similar in articulatory (place of articulation) and acoustic (prevoicing) terms, there are important contrasts associated with these sounds in each language that may affect second language (L2) learners’ acquisition. Spanish maintains an allophonic alternation between word-initial voiced stops and intervocalic voiced approximants, which involves a variation in manner of articulation. Italian maintains a phonemic contrast between intervocalic voiced singleton and geminate stops, which involves a variation in duration. Given these differences, the present study investigates whether the sounds associated with the allophonic alternation in Spanish or those associated with the phonemic contrast in Italian are acquired more easily by L2 learners of each language who share the same L1 (American English) via production and perception tasks. Students enrolled in first-, third-, and fourth-year courses, at the same university, in their respective L2 of Spanish or Italian were recruited for the study. 23 L2 Spanish learners, 20 L2 Italian learners, and five native speakers each of Spanish and Italian participated in the study. Production was assessed with a reading task, while perception was assessed with discrimination and identification tests. The results of the acoustic analyses indicate that learners struggle to produce target sounds in a target-like fashion, as L2 Spanish learners produced word-initial [b d g] with significantly less prevoicing than native speakers and they infrequently produced target approximants as such. L2 Italian learners struggled to precisely implement the phonetic cues that distinguish geminate stops from their singleton counterparts (e.g., preceding vowel duration and consonant duration). In addition, correlation analyses revealed that L2 Spanish and L2 Italian learners’ production and perception are related, although not strongly. Therefore, it is possible that learners’ production difficulties have a perceptual basis, as L2 Spanish learners struggled to discriminate voiced approximants from voiced stops and L2 Italian learners struggled to identify the length difference between voiced singleton and geminate stops. This finding constitutes a valuable contribution to L2 Spanish and L2 Italian phonology, as the role of perception as a basis for learners’ production difficulty of these target sounds has been understudied and not well-understood.