Introduction: Acute Gastroenteritis (AGE) is a leading cause of illness and death amongst children in developing countries. Electrolyte disturbances play an important role in the associated morbidity and mortality. Aims and objectives: (1) To study the electrolyte changes in moderate and severe dehydration in AGE in children. (2) To study the incidence and clinical features of Hyponatremic dehydration. Materials and methods: A cross-sectional type of observational study of 200 children admitted with AGE with moderate to severe dehydration was conducted at M R Bangur hospital, Kolkata. The cases were divided into two groups, one with Hyponatremia and the other with Isonatremia and the clinical features were compared. Basic crosstabulation and frequency distributions were prepared. Chi-square test and Fisher's Exact test were used to test the association between different variables. Z-test and t-test were used to test the significant differences. P ≤ 0.05 was considered statistically significant. Results: AGE constituted 18% of the total admissions. 22% had Hyponatremia, 71.5% had Isonatremia and 6.5% had Hypernatremia. Out of 30 children who were suffering from Hyponatremic dehydration and had ORS before admission, 83.3% were given diluted ORS. Clinical features significantly associated with Hyponatremia were increased frequency of diarrhea, absence of thirst, tachycardia, abdominal distension and severe dehydration. Conclusion: Hyponatremic dehydration is the second most common type of dehydration next to Isonatremic dehydration, but it is more common in children who took diluted ORS. Hyponatremic dehydration may be suspected from the history and clinical features. Increased awareness regarding ORS preparation may help in preventing Hyponatremia in AGE. What is already known: Isonatremic dehydration is the most common electrolyte abnormality found in AGE. What this study adds: Hyponatremic dehydration is the most common electrolyte abnormality found in those suffering from AGE, who have taken inappropriately diluted Oral Rehydration Solution (ORS).